41. 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 这道题OG的解释里边有一个解释是 conditional verb is needed,这个conditional verb是个啥意思? 或者这个would have toCDE项的那些能帮我解释一下么,谢谢~