I can't find Mary downstairs.She ____ her homework upstairs.A must do Bmust be doing C must have

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

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