单项选择题

What is implied by saying "a layer of soil acts as a sponge" (Para. 2)

A．Soil can be used to clean things.
B．Soil can store water and give it up when needed.
C．Soil is all elastic material.
D．Soil can provide water.
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单项选择题
What does this refer to in the sentence This leaves gaps of bare soil... (Para. 4)
A．Cutting down trees and grasses and burning them.
B．The New Stone Age people's clearing of rain forest.
C．Planting of seeds in the bare soil.
D．This type of farming.
单项选择题
The crops' reception of the minimum of competition from other plants is necessary because ______.
A．other plants competed with crops for sunlight and rain
B．people were farming in Asia, Africa and Europe
C．they could get rid of other plants
D．they wanted keep the biological balance
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