GWD中的问题？

To develop more accurate population forecasts, demographer 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 ad economical

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

我的疑问：为什么能用have to 啊，不是说have to 带有主观色彩要用must 吗？我在石林语法上看到是这么写的啊？

还有为什么要用would have to 为什么要这个would啊？

那have to呢？不是表示主观色彩么？也就是这个是不成立的喽