Question

My sister didn't go to the party last night because she ________ the baby for her friend until 9:30.

1. A.
   must have looked after
2. B.
   would have to look after
3. C.
   had to look after
4. D.
   should have looked after

Answer 1

C
该句由because引导的主从句相成，前句表示后果，后句表可原因，与前句时态照应，所以用一般过去时。