9.     Migraine, the most debilitating common form of headache, afflicts perhaps 18 million Americans, who collectively lose 64 million workdays a year, and they cost the nation $50 billion in medical expenses and lost work time.

(A) year, and they cost the nation $50 billion in medical expenses and lost

(B) year and thus cost the nation $50 billion in medical expenses and lost

(C) year, so as to cost the nation $50 billion in medical expenses and lost

(D) year that costs the nation $50 billion in lost medical expenses and（B）

(E) year, which thus cost the nation $50 billion in lost medical expenses and

这道题C为何不对呢？

505, sec20-09

只有B可被接受。有thus, 使cost和lose紧相连, cost 的逻辑主语是Americans, 为一般现在时, a year因为and的存在同样作用于cost(这样理解意思上更清楚)。cost可见的例句中主语都不是人发出的, 这里意思上说: 人(或是就把Americans当作一个东西看)在某方面耗费了国家钱，勉强也能说得通。cost用法还可参见og110。

C的 so as to多用于系表结构中。这里so as to可认为表达复杂, 且不能确定是Migraine cost还是Americans cost。
cost补充例句

nearly 130 million Americans are overweight or obese – a condition that cost the nation $117 billion in medical and other expenses in 2000.

It is estimated that cardiovascular diseases will cost our nation more than $368 billion in 2004

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I  got  it ,thanks !!!!!