单选题 Why are two shots thought to be necessary? People born after 1957 have not been exposed to a similar virus. The first dose "primes" the immune system to respond, but it's the second shot that triggers the production of antibodies. This is not unusual. A number of vaccines are given for the first time in multidose series. For example, the seasonal flu vaccine is given in two shots to children under 9 if they've never had the flu.
Why are two shots thought to be necessary?

• A. Because most people have not been exposed to a similar virus.
• B. Because one shot is enough to trigger the production of antibodies.
• C. Because flu vaccine has always been given in two shots.
• D. Because most people have never had a flu.