问题
单项选择题
Someone is knocking at the door. It ______ my mother. It's time for her to be back.
A. can be
B. may not be
C. must be
D. mustn't be
答案
参考答案：C
解析： 情态动词中的must，can，could，may都表推测。其中must的可能性最大，can次之，may最小。由该句话的语境可以推断出，一定是我妈妈回来了。故选C。