Question

The world's population has grown to more than six times____it was in 1800. A.that B.what C.which

D.when
求解释！

Answer 1

这是一个主句和一个从句的关系，主句The world's population has grown to more than six times，从句it was in 1800，单看从句，句子完整不缺成分，肯定就不能选that/what/which，只有选一个时间状语。所以选when

Answer 2

The world's population has grown to more than six times what it was in 1800.
答案是B

目前的世界人口与1800年相比已经增长到那时的六倍多！

短语grow to意思是“增加到”。类似的表达法还有increase to, rise to, climb to等。如：

Answer 3

选择B项，主要是当你问人口有多少的时候，我们会运用What is the population？句子中it指的是the world's population,整句话的意思：与公元1800年的人口相比，现在的人口是原来的六倍多。