求助NN,would的用法和一般条件句

帮mm补充一下题目。

187.
            (32525-!-item-!-188;#058&007080)     (GWD-1-Q32)

To 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 economic

这题似乎would不是考点，用那个动词形式都可以。问题是其他选项都有错误。

 You would have finished it if you had tried. 
 你果真去做的话，你早就完成了。

那这个来说吧，(想象一个老妈教训孩子的场景)你做了么？没做，所以if中的情况是个虚拟，因为和事实相反 if还有另外个情况，if it rains tomorrow, i will stay home.这个就不是个虚拟，因为明天下不下雨不清楚，仅仅是个条件。
would have 应该是个固定用法，要是用直接用have 那么“你完成了”也就是事实了，而事实上你根本没有完成，因为你没有去做