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

1. have to know a great deal more than now about the social and economic
2. have to know a great deal more than they do now about the social and economical
3. would have to know a great deal more than they do now about the social and economical
4. would have to know a great deal more than they do now about the social and economic
5. would have to know a great deal more than now about the social and economical

The KEY is E, why yah? :-( My answer is B.

would have to好像是必需的，最近练作文发现的规律。

答案是D吧。。。就是D，LZ再看看答案

economical意思是“节俭的”，economic是“经济方面的”