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.
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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.
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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