[求助][讨论]XDF P 104-17

17. 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.

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

答案B

提问：

1.　cost的主语是什么？18 million Americans　吗？cost 是动词现在时还是过去时？

2.　lost 的主语也是18 million Americans　吗？lost 怎么用了过去时？

3.　C的错误是不是应该纠正为：so at to cost .......and （to) lose  就对了呢？

谢谢指教！

没人回答吗？等待中。。。。。。

主语是18 million Americans　，cost 是动词现在时。这里没有任何地方提到了过去的时间。lost这里是形容词，意思是失去的工作时间。

C的错误在于so as to,使得cost主语变成了Migraine,不符合逻辑关系。