--- Look, the light in the teacher's office is still on. Mr Li is working there. He always works late at night.--- It ________ be Mr Li. He has gone to London for a short visit.A.can'tB.mustn'tC.mayD.must
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A解析点拨：此题考查情态动词表猜测。据题意知，既然李老师去伦敦渡短假了，所以那人不可能是他。首先，空缺处应填否定式，所以C和D先排除；mustn't表示“一定不要”，can't表示“不可能”，指心理猜测，符合题意。