GWD-1-Q32 题外话

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

D is the correct answer