It's dark Mum be waiting for my return.A. canB. shouldC. mustD. may

问题补充：

Its dark, Mum be waiting for my return.

A. can

B. should

C. must

D. may

答案：

A. can

B. should

(答案→)C. must

D. may

解析：must表示一种推测,意为:想必, 一定.只能用于肯定句中.