[单项选择]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 vaccine.