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  3. During its growth, a large tree takes in with the help of its roots many tons of water. Yet the tree keeps only very little of t

During its growth, a large tree takes in with the help of its roots many tons of water. Yet the tree keeps only very little of t

admin2009-05-15  66
问题     During its growth, a large tree takes in with the help of its roots many tons of water. Yet the tree keeps only very little of the water, perhaps one percent. The rest is given off through its leaves into the air.
    When we think of a whole forest with so many trees, the total must clearly be very great, even though the amounts may be different according to the kind of tree.
    A tree can be seen as a kind of pump (水泵) in two ways. It is always sucking up water from the ground, which would otherwise become far more loosely made up. At the same time, it increases the humidity(湿度) of the surrounding land, which would otherwise be much drier.
    Trees and forests also act as a breakwater(防水堤) against rainstorms, which, especially on hillsides, would otherwise wash away much of the topsoil, both around the trees and lower down. The rich humus(腐殖土) that forms the floor of the forests acts as a sponge(海绵). It holds far more water than the soil with little humus.
选项 A、roots
B、leaves
C、trunk
D、branches
答案A
解析
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