题目

【2011·重庆主城八区调研】 A Suddenly all the lights on that crowded underground train went out. Then it stopped in that dark tunnel(隧道). We waited. We expected the lights to come on again soon. But nothing happened. Then I began to smell something. Yes, I could smell smoke. So I asked myself, “Is this train on fire?” People around me then began to cry out in fear. “The train is on fire!” they cried. Then the woman standing next to me suddenly fell on the floor. I realized she was overcome by the smoke. Then I saw flames appearing. “If something does not happen 预防接种可以有效地预防传染病，其原因是（ ）A．激活人体内的吞噬细胞B．使人在不发病的情况下产生抗体C．能促进新陈代谢，增强抵抗力D．直接消灭侵入人体内的病原体