单项选择题

Why is acid rain little understood______

A．Because it is a new phenomenon
B．Because it is not frequent enough
C．Because its formation is complex
D．Because it is difficult to study it
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单项选择题
From the first sentence of the passage, we learn that______
A．acid rain has caused air pollution
B．acid rain has caused widespread environmental damage
C．acid, rain has become a much studied subject
D．acid rain travels in all directions before it rains down
单项选择题
Governments_______
A．deny that industries causes acid rain
B．admit that they are only partly responsible
C．claim that they are the main sources of pollution
D．think that the cause of acid rain needs further study
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