[求助]新PREP SC2-187

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

key: D 

这里为什么要用would呢? would在这里表示什么意思啊?