He must be helping the old man to water the flowers ？A. is heB. isn't heC. must beD. must

问题补充：

He must be helping the old man to water the flowers，？

A. is he

B. isn he

C. must be

D. mustn he

答案：

A. is he

(答案→)B. isn he

C. must be

D. mustn he

解析：must在此表猜测，是“一定”之意，此时反意疑问句部分以must之后的动词为准，所以B项正确。