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" (使准备好) 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.
What is the function of the first shot of the vaccine?

• A. To get the body ready for the second shot.
• B. To trigger the production of antibodies.
• C. To get the immune system ready to respond.
• D. To avoid unusual side effects of the vaccin