单选题
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
【正确答案】
C
【答案解析】
第三段第二句指出，由于大气中酸雨形成的化学反应很复杂，几乎不为人们所了解。
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