问题补充：
It is not clear the effects of global warming will be.
A. how
B. which
C. that
D. what
答案：
A. how
B. which
C. that
(答案→)D. what
解析：考查名词性从句的主语从句。本题的it是形式主语，真正的主语是what the effects of global warming will be. what在句中做表语。句意：全球变暖的影响是什么现在还不清楚。故D正确。
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