Only those knew well could be let in.A. whoB. whichC. whatD. he

问题补充：

Only those knew well could be let in.

A. who

B. which

C. what

D. he

答案：

(答案→)A. who

B. which

C. what

D. he

解析：考查定语从句。当先行词是those指人时，且关系代词在句子中做主语时，使用who.句意：只有那些很了解的人才可以被放进来。