Modern science has given clear evidence smoking can lead to many diseases.
A. what B which C. that D where
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解析：考查同位语从句。本题中的同位语从句that smoking can lead to many diseases解释名词evidence的内容，that在句中不充当任何成分，没有任何实际意义，也不能省略。句意：现代科技已经给出了很多的吸烟可以导致疾病的证据。ABD三项都要在句中充当句中成分。故C正确。