一个很难的高中英语单选题----高手帮忙
In my opinion, life in the twenty-first century is much easier than _____.
A：that used to be B：it is used to C：it was used toD：it used to be
答案是D,但是我看不懂D项,it是代指life的对吗?重点说一下it used to be 后面是不是省略了东西?如果是完整的,我怎么也理解不了,怎么可能以be结尾呢?