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 請問這題為什麼要用would呢? economic或economical在這題也是考點嗎? 感謝~ --

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