单项选择题
A．How trees help prevent droughts and floods. B．The relationship between trees and man. C．How an empire fell to pieces in ancient times. D．A story of trees.
A．Because nothing remains on land except floods. B．Because there are no longer trees to keep the rain and protect the top-soil. C．Because too much rain sinks in and washes away the top-soil. D．Because roots of the trees break up the soil.