How do vaccines protect humans from diseases according to paragraph 2? A．By training the immune system to fight weaker versions of the disease. B．By passing information on how to fight the disease to the immune system. C．By weakening the disease so that the immune system can defeat it. D．Introducing the disease to the body, so that survivors have already fought it.
A.By B. B．By C. C．By D. D．Introducing E.
【参考答案】
A 属事实细节题。相关信息在第二段第一、二句：接种疫苗是通过将弱小或死亡的菌株注射入监控环境下的病人体内，让人体......