[求助]prep 2#187，请进～

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

我选的是A，答案为D，为什么呢，would家在这里是什么意思呢？