The librarian requires that all books ______ returned before the last day of classes.

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.

A．How trees help prevent droughts and floods.
B．The relationship between trees and man.
C．How an empire fell to pieces in an ancient times.
D．A story of trees.