[求助]抓狂，省略问题，跪求详解

两道题：

GWD1-Q16:

Soaring television costs accounted for more than half the spending in the presidential campaign of 1992, a greater proportion than it was in any previous election.

A.    a greater proportion than it was

B.    a greater proportion than

C.    a greater proportion than they have been

D.    which is greater than was so

E.     which is greater than it has been

To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.

A.    have to know a great deal more than now about the social and economic

B.    have to know a great deal more than they do now about the social and economical

C.    would have to know a great deal more than they do now about the social and economical

D.    would have to know a great deal more than they do now about the social and economic

E.     would have to know a great deal more than now about the social and economical

自己看着这两题真是像啊。但是为啥GWD1-Q16中B项中的省略就可以，GWD1-Q32中的A为啥就不能理解成have to know a great deal more than （they do） now about the social and economic呢？

省略问题真的很让我崩溃，NN一定要帮我啊！