单项选择题

In a "primary succession", what makes it possible for mosses to take root

A．The type of rock.
B．The amount of sunlight.
C．The amount of moisture.
D．The effect of lichens.
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单项选择题
What has sometimes caused plants to die out
A．Interference from foresters.
B．Variations in climate.
C．The absence of wooded land.
D．The introduction of new type of plants.
单项选择题
In what way does the forest strike mankind as permanent
A．The trees are in community.
B．The forest is renewed each season.
C．Man's life is short in comparison.
D．It is an essential part our lives.
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