We don’t know the reason he didn’t accept the job. A. for whatB. for thatC. for whichD. f

问题补充：

We don’t know the reason he didn’t accept the job.

A. for what

B. for that

C. for which

D. for why

答案：

A. for what

B. for that

(答案→)C. for which

D. for why

解析：考查定语从句。for which= why，当先行词为reason时。如果前面有介词，引导词只能使用which，故选C。