Tom`s mother was always telling him what he should do,but( )didn`t help.

问题描述:

Tom`s mother was always telling him what he should do,but( )didn`t help.

答案:it
解析:如果从句子的角度看来这个空应该代指前面的句子,可以考虑it和which,但which是从属连词,引导定语从句,前面不能有并列连词but,很明显应该填it
补充:如果有选项的话,用this或that也勉强可以算做正确.