has been said before it is not easy to learn a foreign language.A. AsB. ItC. WhatD. As it

问题补充：

has been said before, it is not easy to learn a foreign language.

A. As

B. It

C. What

D. As it

答案：

(答案→)A. As

B. It

C. What

D. As it

解析：as表“正如”，引导非限制性定语从句，代替后面整个主句的意思。