31. ---Do you want a lift home? ---It’s very kind of you, but I have to work late in the office. I overslept this morning because my alarm clock _______.
(A) doesn’t go off
(B) won’t go off
(C) wasn’t going off
(D) didn’t go off

本题考查时态的用法。根据I overslept this morning 可知应用一般过去时。