. Research findings show we spend about two hours dreaming every night, no matter what we

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A. should
B. would
(答案→)C. may
D. must
解析：考查情态动词。句意“研究结果显示，我们每晚要花两个小时的时间做梦，无论白天做了什么事情。”should have done“本应该做某事”；would have done用于对过去事实的虚拟；may have done“可能已经做了某事”；must have done“一定已经做了某事”。这里表示“可能”。故选C。