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.
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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.
单项选择题
Why is a secondary succession quicker
A．The ground is more suitable.
B．There is more space for new plants.
C．Birds and animals bring new seeds.
D．It is supported by the forest.
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