大全505---section20-09 求证+求助

505.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
(E) year, which thus cost the nation $50 billion in lost medical expenses and  （B）

我选择了A

思路是：（1）忠于原文-------CD中lost的位置变化。句意变。所以排除。

              （2）so as to结构只用于简单的主系表结构中。目前做的题中多以so。。。that句型代替。所以将c排除。

              （3）剩下A和B。不知道进一步判断的一句了。就忠原文。猜了一个A。

不知道这个思路对吗？谢谢

弱问几个问题： 

（1）lost在这里是形容词吗？形成小并列 in medical expenses and lost work？

（2）原文中cost the nation $50 billion 只是一个顺承的附属关系。如果用and they 则提到了一个与主句相当的位置上。所以A错吗？

（3）若用so as to，则这里的主语应该是通主句主语migraine？还是americans呢？

（4）A中they的指代是指？个人认为是americans。

还请指点。再次谢谢。

谢谢。

不过对于第二个问题还是不明白。

如果A中的they指代错误。那么那B中，

and thus cost the nation $50 billion 的主语是谁？不是americans吗？如果是migraine的话。cost ---> costs？

好像american是可以做cost的主语。以下是翻出的以前的帖子。