His father often tells him in that river because it’s dangerous.A. not to swimB. not swim

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(答案→)A. not to swim
B. not swim
C. swim
D. to swim
解析：不定式的否定式通常是在不定式符号to前面加not，tell后接带to的不定式。根据下文因为太危险了。前文意思为：他的父亲告诉他不要去在那条河里面游泳。故选A。