Question

________ is important to master at least a foreign language in modern times especially after China’s entry WTO．

1. A.
   That
2. B.
   What
3. C.
   This
4. D.
   It

Answer 1

D
本句子的真正主语是to master…，it在此作形式主语。形式主语或形式宾语只能用it。