XDF蓝皮中册P104T17???

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 medical expenses and

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

WHy answer is B?? shouldnt it be " costs" rather than "cost"??