Question

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

1. A.
   did I realize
2. B.
   I realized
3. C.
   had I realize
4. D.
   has I realized

Answer 1

A
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。