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题库 / Prep2008E2-SC-187
To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.
have to know a great deal more than now about the social and economic
have to know a great deal more than they do now about the social and economical
would have to know a great deal more than they do now about the social and economical
would have to know a great deal more than they do now about the social and economic
would have to know a great deal more than now about the social and economic
题目分析：
本题具有比较词more…than…。
选项分析：
A选项：比较对象不正确，我们应该比较的是不同时间的两件事，而不是两个时间（now）。
B选项：economic和economical的意思不同，前者是“经济”的意思，后者是“节约的”意思，在逻辑上，和“社会(social)”产生并列关系的应该是“经济”，而不是“节约的”。
C选项：economic和economical错误同(B)。
D选项：Correct. 本选项在语法和逻辑上均是正确的。
E选项：比较对象不正确错误同(A)。
状语比较的时候，可以省略than后面整个主句，不管时态是否一致。
Eg:
Scientists have gathered evidence suggesting that complex life-forms emerged much earlier than previously thought.
than previously thought (when they emerged)
时间状语的比较，能否省略取决于than前面是否有相应的时间状语。有则than后面主句可以省略；没有则than后面需要补出主谓，以避免比较对象上的歧义。
Eg:
Right: Prices are only 1.3 percent higher this year than last year.
Wrong: Prices are only 1.3 percent higher this year than last year's.
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