They are sweating all over. They ______ in the field.

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问题 They are sweating all over. They ______ in the field.

选项 A、may be
B、must have been working
C、could be
D、must having been working

答案B

解析 情态动词可和一个动词的完成进行式构成谓语,表示“想必一直在,应当一直在”等意思。本句must表示肯定推测,同时根据句意,劳动一直在进行,故B项must have been working正确。
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