It is global warming, not other things, the extreme weather.
A. that have led to
B. which has caused
C. which are causing
D. that has led to
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A. that have led to
B. which has caused
C. which are causing
(答案→)D. that has led to
解析：考查强调句型。强调句型的基本结构为：it is/was被强调成分that/who其它成分；最大的特点是把it is…that/who…去掉，句子仍然成立。本题强调的是主语global warming.句意：正是全球变暖而不是其它的事情导致了极端天气。故D正确。