请教GWD-XY第一套里的一道语法题

siying750

原题如下：
üTo 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

我选的是D，但是答案显示是E.
为什么要用economical呢？那不是节约的意思么？
另外，more than they do now里的 they do 可以省略的吗？
谢谢！！

9car

毫无疑问，此题选D，楼主正解，OG 12版原题，题号在50-80之间