单项选择题

The Para 5 mainly discusses ______.

A．how animal wastes enriched land
B．how plant roots protected the land
C．how humus becomes useful
D．how a forest creates its own soil and protects the watershed

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单项选择题

Once upon a time, the United States seemed to have plenty of land to go around, go around means ______.

A．to visit around
B．to see the land
C．for sharing with every
D．to have the land around