求助各位高人为小弟指点GWD0132(抱歉找不到对应在大全上的编号)

mcyuyu

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

   正确答案是啥and为啥？主句是否要加would?  这里的have是不是助动词？ 比较句than后是否加do？

wanze

d