All this leads to a population in the twenty-first century that is smaller______ years ago.
问题描述：
All this leads to a population in the twenty-first century that is smaller______ years ago.
A.than was B.than that C.that was D.it was 为什么选择A ,这是一种什么用法呢?
答
所有这些（All this ）导致21世纪的人口比三年前的（人口）更少了.
中文翻译是这样的,其实并没有什么特殊的用法,就是省略了the population.
All this leads to a population in the twenty-first century that is smaller than(the population,或者用it代替the population）was three years ago.所以选A