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Xiao li seldom does exercises so he is not strong.

Being lack of enough exercises, xiao li doesn't look so strong.

Lee（xiao li）rarely does exercise， and so she/he is not fit (enough). 这里用strong会有点搞笑。。其实指的是身体不够健康容易得病。

Xiao Li is not strong enough for he seldom does exercides

Xiao Li is not strong enough for lacking sports.