[求助]GWD-1-32

To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.

1. have to know a great deal more than now about the social and economic


2. have to know a great deal more than they do now about the social and economical


3. would have to know a great deal more than they do now about the social and economical


4. would have to know a great deal more than they do now about the social and economic


5. would have to know a great deal more than now about the social and economical

答案是E，但我不明白C错在哪里？尽量补出谓语动词或助动词不是比E更好吗？

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