单项选择题


A．harmful
B．profitable
C．healthy
D．useful
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单项选择题
What has sometimes caused plants to die out of the past
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
Why is a secondary succession quicker A. The ground is more suitable. B. There is more space for new plants. B. Birds and animals bring new seeds. D. It is supported by the forest.
A．Birds and animals bring new seeds.
B．It is supported by the forest.
C．There is more space for new plants.
D．Why is a "secondary succession" quicker
E．The ground is more suitable.
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