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What conditions are needed for shrubs to become established

A．Ferns must take root.
B．The ground must be covered with grass.
C．More soil must accumulate.
D．Smaller plants must die out.
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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.