语法题语意的理解

oddyuibe

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.)

the answer is (a year and thus[U] cost [/U]the nation $50 billion in medical expenses and lost work time.)

到底是什么导致了500亿美元的医疗费用呢？用cost表明主语是Americans，美国人导致了医疗费用，这个语义上能讲的通嘛？

谢谢指点

maryland

在and thus cost 之前没有逗号吗？按照我的理解，应该是migraine cost the nation $50 billion in medical expenses and lost work time。由于用了lost修饰work time，这里的cost可以理解为过去时。

请大家讨论。[em09]

oddyuibe

to maryland,

and thus cost之前没有逗号

请讨论：）