I don’t know the reason he’s come so late.A. /B. for whichC. whyD. all the above

问题补充：

I don’t know the reason he’s come so late.

A. /

B. for which

C. why

D. all the above

答案：

A. /

B. for which

C. why

(答案→)D. all the above

解析：reason后面定语从句的引导词是why，that，for which或不用引导词。