单选题
Mary has got a furl mark in the test. She ______ very hard all the time.
A、
will work
B、
must have worked
C、
may work
D、
could have worked
【正确答案】
B
【答案解析】
[解析] must have done表示对过去情况的推测，例如：Her eyes are red. She must have been crying。(她的眼睛红了，一定是哭过了。)D项could have done译成“本可以”，不合题意，故选B。
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