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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)。
如果要用助动词来省略性表达前面出现过的一个谓语动词，被省略句子必须与前面句子的时态一致。如果两者的时态不一致，后面的从句不能用省略表达。
eg.:
Right: I have never seen an aardvark, but my father has.
Wrong: I have never seen an aardvark, but last year my father have.
Right: I have never seen an aardvark, but last year my father saw one.
除非不会导致歧义，如：
Our cars were designed to inspire envy, and they do.
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