Sentence Correction

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

OA is B

Dear All,

I don't understand here why "cost" is not "costs" or "to cost"?

I believe the subject is the Migraine. However, if the answer is correct, the subject is "The Americans"?

Could, please, anyone bring more explanations?

Thanks a lot in advance

Fred