Among the following ecosystems, () a. Grassland plays an important role in water conservation, soil and water conservation, wind and sand fixation, climate regulation and air purification.

A. Grassland ecosystem: It is distributed in areas with drought and little rain, short of tall plants and few species of animals and plants.

B. Forest ecosystem: mainly distributed in humid and semi-humid areas, with a wide variety of animals and plants. It is the region with the richest biodiversity in the earth environment. Forests play an important role in water conservation and soil and water conservation, and are called green reservoirs. B is right.

C. Wetland ecosystem: it is an ecosystem formed under the condition of excessive moisture and humidity. Marsh is a typical wetland ecosystem, with marsh plants as the dominant species and various animals. Wetland has the function of purifying water source, storing flood and fighting drought, but it does not have the function of conserving water source and soil and water.

D. Lake ecosystem: It belongs to fresh water ecosystem, and refers to the ecosystem formed by the interaction between fresh water biota and fresh water area, which can be divided into still water and living water. The former refers to freshwater lakes, ponds and reservoirs. The latter refers to rivers, streams and canals, which are easily damaged and difficult to recover.

Therefore, choose: B.