Mary her mother with the housework but she didn’t.A. must have helpedB. ought to have he

问题补充：

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

答案：

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。