It isn’t clear is to blame.We need to find a solution to the problem first.A. who is it th

问题补充：

It isn’t clear is to blame.We need to find a solution to the problem first.

A. who is it that

B. who it is that

C. it is who that

D. that who it is

答案：

A. who is it that

(答案→)B. who it is that

C. it is who that

D. that who it is

解析：这里It是形式主语，who引导的是真正的主语从句，从句是强调句型，被强调部分是who，因为who是连词必须放在主语从句的句首，又因为主语从句的语序是陈述语气。故选B。温馨提醒：强调句型的结构：it be 被强调部分+ that+其他，判断强调句型的方法是把it be that去掉，整个句子是成立的。