<em>Vaccines work by simulating an illness to help establish immunity. However, while this sort of infection nearly never produces sickness, it does trigger the immune system to create T-lymphocytes and antibodies. The imitation illness can sometimes induce modest symptoms, like as fever, after receiving a vaccination.</em>
Explanation: Vaccines help develop immunity by imitating an infection. This type of infection, however, almost never causes illness, but it does cause the immune system to produce T-lymphocytes and antibodies. Sometimes, after getting a vaccine, the imitation infection can cause minor symptoms, such as fever.
George is running a 5-km race and his cells need more oxygen. The change that will help his body meet the increased demand for oxygen is his heart will start beating more quickly. This helps the oxygen get ferried throughout the system faster. The correct answer is A.
