GWD13-30

For the first time in the modern era, non-Hispanic Whites are officially a minority in California, which amounts to a little less than half the population of the state, down from nearly there-quarters only a decade ago.

A.    which amounts to a little less than half the population of the state, down from nearly three-quarters only a decade ago

B.    which amounts to a little less than half the population of the state, down from a decade ago, when it was nearly three-quarters

C.    and that amounts to a little less than half the population of the state, down from a decade ago, when they were nearly three-quarters

D.    amounting to a little less than half the population of the state, down from nearly three-quarters a decade ago

E.     amounting to a little less than half the population of the state, down from what it was a decade ago by nearly three-quarters

answer is D why C?

thank you!

如果that是引导子句则前面没有可以平衡的对象

如果that是代名词的功能的话，没有指代的对象