There are some students to have known what they don’t really know, which is really harmfu

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(答案→)A. pretending
B. to pretend
C. pretended
D. pretend
解析：考察分词作定语。现在分词短语Pretending to have known what they don’t really know作定语修饰some students。因为some students与pretend关系主动关系，使用现在分词表示主动。To pretend表示将来的动作，故不合适。