The most important thing we should consider is the first idea he has talked about in the

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A. which; that
B. that; which
C. which; which
(答案→)D. that; that
解析：这题考查定语从句的用法：第一空先行词是最高级 the most important thing，所以用that引导定语从句，第二空还是填that，，因为先行词被序数词the first修饰，选D。