单选题
That a different component part of the vaccine is necessary is principally due to the variable change of ______.
【正确答案】
C
【答案解析】文章第二段指出“constant changes of antigenic specificity of the virus necessitate a different composition of the vaccine(疫苗)from one year to another”，即病毒抗原特异性的不断变化导致了疫苗组成年年不同。故答案为C。