【单选题】Shortages of flu vaccine are nothing new in America, but this year's is a whopper. Until last week, it appeared that 100 million Americans would have access to flu shots this fall. Then British author...
A.
America relies too much on foreign suppliers
B.
the demand of flue vaccines is high this year
C.
quality problem is a serious problem in flu vaccine production
D.
the supply of flu vaccines is rather weak and America has no back-up measures to make it up