To develop moew 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 答案是D 请问这题为什麽要用would呢? economic或economical在这题也是考点吗? 感谢~ --

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