求助xy13上的一道语法题

alexis_ye

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

正确答案是D。

 

我的问题是A选项错误的原因是它没有使用虚拟语态还是than后面没有对称地使用they do这个结构？

 

另外，想问问than这个词的用法。究竟than后面什么成份是可以省略的，什么是必须有的？

 

还请nn们不吝赐教。。。谢谢啊！

[此贴子已经被作者于2008-1-30 22:57:04编辑过]

plato1983

A选项错误在于than前后比较的对象不对称。

足球幽灵

than只要不造成歧义，就可以省略。A项错就是因为than后的省略造成歧义：1，D要知道的不只是S&E（D要知道更多除S&E外的其它因素）；2，在S&E方面，D要知道的更多。

小W

这道题would是否可要可不要，如果后面的语法都对的话？