Often conjuring images of dank, smelly, mosquito-infested wastelands, upon closer look, wetlands are actually biologically diverse and productive ecosystems. Home to a variety of plant life, including floating pond lilies, cattails, cypress, tamarack, and blue spruce, wetlands support diverse communities of invertebrates, which in turn support a wide variety of birds and other vertebrates. Primary consumers from crustaceans, mollusks, and aquatic insect larvae to muskrats, geese, and deer rely on the abundance of algae, plants, and detritus for food. Wetlands also support a variety of carnivores, including dragonflies, otters, alligators, and osprey. Thus, wetlands of the world maintain biologically diverse communities of ecological and economic value. Many ecologically and economically important species call wetlands home for at least part of their lives. For instance, commercially important fish and shellfish, including shrimp, blue crab, oysters, salmon, trout, and seatrout rely on, or are associated with, wetlands. Wetlands are also critical habitat for migratory birds and waterfowl, including ducks, egrets, and geese. In fact, more than one-third of the species listed as threatened or endangered in the United States live solely in wetlands and nearly half use wetlands at some point in their lives. As such, many wetlands are often recognized as important conservation or restoration targets. While covering only six percent of the Earth’s surface, wetlands provide a disproportionately high number of ecosystem services, in addition to maintaining biodiversity. For instance, wetlands also mitigate floods, protect coastal areas from storms, improve water quality, recharge groundwater aquifers, serve as sinks, sources, or transformers of materials, and produce food and goods for human use. When evaluating the economic value of these various functions, scientists concluded that the economic value provided by wetland ecosystems exceeded that provided by lakes, streams, forests, and grasslands and was second only to that provided by coastal estuaries. With mitigation, wetlands are created, restored, or enhanced to replace wetland loss due to development. The Ramsar Convention, an international treaty aimed at conserving wetlands, requires member countries to develop national wetland policies, to establish wetland reserves, and to designate one or more wetlands as an area of international importance. All these efforts are designed to protect or conserve wetlands and the ecosystem services they provide. In coastal wetlands, tidal influence drives the movement and distribution of water and can range from permanent flooding in subtidal wetlands to less frequent flooding in others, with changes in water level occurring daily or semi-daily. Inland wetlands, which lack daily tidal influences, can also be permanently flooded on one extreme or intermittently flooded on the other extreme, with fluctuations over time often occurring seasonally. It is the balance of water inflows and outflows, or the water budget, as well as the geomorphology and soils that determine the timing, duration, and patterns of flooding in a wetland.

Of all the ecosystem services wetlands provide, the most important one is______.

A．producing food for humans

B．maintenance of biodiversity

C．improvement of water quality

D．refilling groundwater reservoirs

According to the passage, a wetland is a/an______.