问题补充：
Who do you think for the failure of their marriage?
A. to blame
B. to be blame
C. is to blame
D. is to be blamed
答案：
A. to blame
B. to be blame
(答案→)C. is to blame
D. is to be blamed
解析： do you think 为插入语，who is to blame是句中主干，to blame不定式作表语，用主动表被动。
