应该是It's necessary for keep fit.为什么正确答案确是It's necessary to keep fit.

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应该是It's necessary for keep fit.为什么正确答案确是It's necessary to keep fit.

应该是It's necessary to keep fit 啊.
因为to后面跟动词原形,而for后面只能跟现在分词,而且习惯用法也是用necessary to do sth.而不是necessary for do sth.
to就是引导动词不定式.

这是动词不定式
结构是It's necessary for sb.to do sth.

改成:(T)to keep fit is necessary.
能明白了吗?