单选题：Text 3Shortages of flu vaccine are nothing new in America, b

Text 3
Shortages of flu vaccine are nothing new in America, but this year' s is a whopper. Until last week, it appearedthat 100 million Americans would have access to flu shots this fall. Then British authorities, concerned about quali-ty-control problems at a production plant in Liverpool, barred all further shipments by the Chiron Corp. Overnight,the U. S. vaccine supply dwindled by nearly half and federal health officials found themselves making an unusualplea. Instead of beseeching us all to get vaccinated, they' re now urging most healthy people between the ages of 2and 64 not to. "This reemphasizes the fragility of our vaccine supply, " says Dr. Martin Myers of the National Net-work for Immunization Information, "and the lack of redundancy in our system. "
Why is such a basic health service so easily knocked out? Mainly because private companies have had little in-centive to pursue it. To create a single dose of flu vaccine, a manufacturer has to grow live virus in a 2-week-oldfertilized chicken egg, then crack the egg, harvest the virus and extract the proteins used to provoke an immune re-sponse. Profit margins are narrow, demand is fickle and, because each year' s flu virus is different, any leftovervaccine goes to waste. As a result, the United States now has only two major suppliers (Chiron and Aventis Pas-teur)--and when one of them runs into trouble, there isn' t much the other can do about it. "A vaccine maker can' tjust call up and order 40 million more fertilized eggs, " says Manon Cox, of Connecticut-based Protein SciencesCorp. "There' s a whole industry that' s scheduled to produce a certain number of eggs at a certain time. "
Sleeker technologies are now in the works, and experts are hoping that this year' s fiasco will speed the pace ofinnovation. The main challenge is to shift production from eggs into cell cultures--a medium already used to makemost other vaccines. Flu vaccines are harder than most to produce this way, but several biotech companies are nowpursuing this strategy, and one culture-based product (Solvay Pharmaceuticals' Invivac) has been cleared for mar-
keting in Europe. For Americans, the immediate challenge is to make the most of a limited supply. The government estimates that95 million people still qualify for shots under the voluntary restrictions announced last week. That' s nearly twice thenumber of doses that clinics will have on hand, but only 60 million Americans seek out shots in a normal year. Infact, many experts are hoping the shortage will serve as an awareness campaign--encouraging the people who reallyneed a flu shot to get one.
根据以上材料，回答题。
Shortages of flue vaccine show that__________. A．America relies too much on foreign suppliers
B．the demand of flu 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