问题补充：
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
答案：
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。
