that 和 it 代不可数名词区别在哪里

题目

that 和 it 代不可数名词区别在哪里
in my opinion,life in the twenty-first centry is much easier than————
A that used to be .B.it is used to.
C.it was used to D.it used to be.
选D是为什么呢？为什么A就不行呢？

答案

D
life in the twenty-first centry is much easier than it used to be.
life in the twenty-first centry is much easier than that in the twentieth.
原因我说不上来,只是感觉,但是不会错.以上两句你比较,都是正确的句子.
English can be learnt.It cannot be studied.