1. Tom's mother kept telling him that he should work harder , but           didn't help .

    A. he          B. which           C. she          D. it

〖答案与简析〗选D。测试 it 用作人称代词，代替前文提到过的事物。又如：

1）― The light in the office is still on.

     ― Oh , I forgot            .

     A.  turning it off     B. turn it off     C. to turn it off    D. having turned it off

2）― Have you heard the news about Tom ?

  ― No , what          ?

     A. is it           B. it is          C. are they      D. they are

3）If you don't know the word , why don't you             in the dictionary ?

     A. look at it       B. look after it       C. look up it       D. look it up

答案1）― 3）CAD。值得注意的是，由动词和副词组成的短语动词用it 作宾语时，it 应置于动词和副词之间。