单项选择题

According to the passage, the exchange of medical ideas between east and west started ________.

A．thousands of years ago
B．decades of years ago
C．a few years ago
D．after the Second World War
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单项选择题
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.
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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