Mary her mother with the housework, but she didn’t.
A. must have helped
B. ought to have helped
C. must help
D. should help
网友回答
A. must have helped
(答案→)B. ought to have helped
C. must help
D. should help
解析：考查情态动词用法。根据but后面的didn’t,说明本句是对过去情况的虚拟，故使用“情态动词+have done”的形式。Must have done一定做了某事；ought to have done本应该做某事，实际上却未做；CD两项是对现在情况的推测。句意：Mary本应该帮助妈妈做家务的，但是她却没有。故B正确。