At present, we are studying a total of nine subjects, I think is physics.
A. of them the most difficult
B. and the most difficult of which
C. the most difficult of them
D. the most difficult of which
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A. of them the most difficult
B. and the most difficult of which
C. the most difficult of them
(答案→)D. the most difficult of which
解析：如果用并列句就是：and the most difficult of them；用定语从句就是：the most difficult of
which=of which the most difficult句意：现在我们总共学习九门功课，其中最难的我认为是物理。