When the first of the two Viking landers touched down on Mars on July 20, 1976, and began to send camera images back to earth, the scientists at the Jet Propulsion Laboratory could not suppress a certain nervous anticipation, like people holding a lottery ticket that they have a one-in-a-million chance of winning. The first photographs that arrived, however, did not contain any evidence of life. What revealed itself to them was merely a barren landscape littered with rocks and boulders. The view resembled nothing so much as a flat section of desert. The scientists were soon ready to turn their attention from visible life to microorganisms. The twin Viking landers carried three experiments designed to detect current biological activity and one to detect organic compounds, because researchers thought it possible that life had developed on early Mars just as it is thought to have developed on earth, through the gradual chemical evolution of complex organic molecules. To detect biological activity, Martian soil samples were treated with various nutrients that would produce characteristic by-products if life forms were active in the soil. The results from all three experiments were inconclusive. The fourth experiment heated a soil sample to look for signs of organic material, but found none, an unexpected result because at least organic compounds from the bombardment of the Martian surface by meteorites were thought to have been present. The absence of organic materials, some scientists speculated, was the result of intense ultraviolet radiation penetrating the atmosphere of Mars and destroying organic compounds in the soil. Although Mars’ atmosphere was, at one time, rich in carbon dioxide and thus thick enough to protect its surface from the harmful rays of the sun, the carbon dioxide had gradually left the atmosphere and been converted into rocks. This means that even if life had gotten a start on early Mars, it could not have survived the exposure to ultraviolet radiation when the atmosphere thinned. Despite the disappointing Viking results, there are those who still keep the possibility of life on Mars open. They point out that the Viking data cannot be considered the final word on Martian life because the two landers only sampled two limited — and uninteresting — sites. The Viking landing sites were not chosen for what they might tell of the planet’s biology. They were chosen primarily because they appeared to be safe for landing a spacecraft. The landing sites were on parts of the Martian plains that appeared relatively featureless from orbital photographs. The type of Martian terrain that these researchers suggest may be a possible hiding place because active life has an earthly parallel: the ice-free region of southern Victoria Land, Antarctica, where the temperatures in some dry valleys average below zero. Organisms known as endoliths, a form of blue-green algae that has adapted to this harsh environment, were found living inside certain rocks in these Antarctic valleys. The argument based on this discovery is that if life did exist on early Mars, it is possible that it escaped worsening conditions by similarly seeking refuge in rocks. Skeptics object, however, that Mars in its present state is simply too dry, even compared with Antarctic valleys, to sustain any life whatsoever. Should Mars eventually prove to be completely barren of life, as some suspect, then this would have a significant impact on the current view of the chemical origin of life. It could be much more difficult to get life started on a planet than scientists thought before the Viking landings.

Which of the following titles is proper for the passage?

A．Relationship Between Life on Mars and Evolutionary Theory

B．Scientific Achievement of the Viking Mission

C．Possibility of Life Existence on Mars

D．Limitations of Scientific Investigation of Mars

The phrase “holding a lottery ticket“ underlined in Paragraph 1 is used to______.

A．indicate how likely it was that title photographs would show Martian life

B．suggest that scientists thought the camera might not function

C．refute the belief that life may exist on Mars

D．imply that any mission to another planet is a venture

The four Viking experiments were designed to______.

A．prove that meteorites do not strike the surface of Mars as often as scientists thought

B．show that biological activity is completely void of life on Mars

C．determine whether life exists on Mars

D．provide evidence for the existence of nutrients that produce life forms

According to Paragraph 3, which of the following statements is true?

A．Viking experiments were not sensitive enough to detect life on Mars.

B．Organic materials on Mars might have been destroyed by ultraviolet radiation.

C．Microbes from dry Antarctic valleys will reproduce only every few months.

D．Mars had a shallow ocean covering much of its northern hemisphere in the first few hundred million years.

Paragraph 3 provides______.

A．an analysis of a theory set forth earlier

B．a theory about findings presented earlier

C．some evidence supporting a statement made earlier

D．a summation of facts reviewed earlier

It can be inferred from the passage that Mars______.

A．was not able to support any life as the earth is

B．possessed at one time an atmosphere rich in carbon dioxide, but the earth did not

C．accumulated organic compounds from meteorites

D．had an atmospheric layer that protected organic compounds

What did the author emphasize by mentioning the Viking landing sites in Paragraph 4?

A．Uninteresting as the landing sites were, they could have harbored Martian life.

B．The landers’ failure of finding evidence of life does not mean that Mars is devoid of life.

C．The Viking mission was unsuccessful largely due to selection of improper landing sites.

D．Scientists were not expecting to discover life on the Martian plains.

In Paragraph 5, the researchers’ argument that life may exist in Martian rocks is grounded on the fact that______.

A．endoliths are capable of living in the harsh Martian environment

B．life developed in the form of blue-green algae on Mars

C．organisms may adopt identical survival strategies in comparable environments

D．organisms that have survived in Antarctica could survive the Martian environment

The author concludes the passage by stating that______.

A．human beings could eventually discover life on Mars even if there are many difficulties

B．it is a tougher job for scientists to spot life on Mars than they expected

C．the detection of life on Mars was not a primary objective of the scientists who sent the Viking landers

D．Viking landing influences scientific theory regarding the chemical origin of life on the earth

In significant parts of Eurasia, the middle centuries of the first millennium______a significant transition in human cultural history. define mark identify specify

It can sometimes feel like the world is very noisy — from planes arching loudly through the sky to the______buzz of your cellphone. unavailing unassuming unconvincing unceasing

Sub-Saharan Africa and Southeast Asia are most at risk from bat viruses______humans and causing new diseases that could lead to deadly outbreaks. vaulting bouncing attacking hurdling

Poland is somewhat a new destination for Chinese tourists, and Warsaw, the capital city, is an important business center with______night life and elegant dining. sophisticated sluggish sloppy solitary

In this new world the most dangerous conflicts will not be between social classes or other economically defined groups, but between peoples belonging to different cultural______. parts components entities sects

Certain religious and political groups felt novels had the power to make immoral characters so interesting that younger readers would______with them easily. agree comply identify differentiate

The mayor is ideologically consistent. His long-term commitment to education reform, for example, is not a sign of fickleness. coolness carefulness calmness capriciousness

Under the attack of its enemies from all sides, the empire was about to crumble. disintegrate disembark disgrace disengage

Despite her relaxed style, Ms. Flagen is a canny businesswoman who knows how to aggressively market her brand. thrifty shrewd cunning amicable

The problem of avoiding duplicate names, such as for E-mail accounts, is particularly acute when only a finite number of possibilities are allowed. solemn stern stiff severe

The environmentalists’ warning against the blatant pursuit of industrialization without caution seems especially prescient. precarious superficial foresighted mysterious

Britain is attractive to worldwide advertisers because it is a homogenous market, so there is no need to tailor advertisements to different parts of the country. global local foreign uniform

A discovery in New Jersey actually contributed to the early economic development of the U.S. and, in 1714 a worker uncovered a green rock containing copper. when, in 1714, thus, in 1714, it was in 1714 till in 1714

A study found that the environmental costs of producing and recycling paper cups were as high as, if not higher than, to produce and recycle polystyrene cups. those of producing and recycling that of producing and recycling which produced and recycled that produced and recycled

Blue lights flashing and siren blaring, the police car sped through the busy intersection. Blue lights flashed and siren blared To flash blue lights and blare siren With blue lights and siren flashing and blaring With blue lights flashing and siren blaring

Linguistic research often requires a period of fieldwork where the linguist can study and record the spoken dialects of a region. through which during which with which by which

Veterans of World War II received greater support from the general public than later wars. than did than what did than did those of than of those

Volunteering on political campaigns can help students become a more attractive candidate for jobs in government. become more attractive candidates become a candidate more attractive becoming candidates more attractive becoming a candidate more attractive

About 75 percent of all cadmium is used for cadmium plating of metals are easily corroded such as iron and steel. metals easily corroded corroded metals easily corroded easily metals easily corroded metals

Most fishes and many reptiles have ribs along most of the spine, but in mammals finding them only in the chest area. they find which they find they are found which they are found