Not until then their marriage was breaking up because they had little in common.A. did I

问题补充：

Not until then their marriage was breaking up because they had little in common.

A. did I realize

B. I realized

C. had I realize

D. has I realized

答案：

(答案→)A. did I realize

B. I realized

C. had I realize

D. has I realized

解析：考查倒装结构。否定词not until放在句首，主句的主谓要倒装，根据句意用一般过去时，选A。