I can't find Mary downstairs.She ____ her homework upstairs.A must do Bmust be doing C must havedone Dcan't do答案是B,为什么不选A

问题描述:

I can't find Mary downstairs.She ____ her homework upstairs.A must do Bmust be doing C must have
done Dcan't do答案是B,为什么不选A

因为回答的时候Mary正在楼上做作业,所以用进行时。
而A是现在时,不符合语境。

他是说玛丽一定正在做作业,所以我现在找不到玛丽啊.be doing是现在进行时,所以选B更加贴切