题目

       This year the marathon in my hometown took place in the hottest month.My job was to follow behind the runners to see if any of them needed medical care.        “We should stay behind the last runner, so take it slowly，” I said to the driver, Bob.        “Let's hope that the last runner is fast！” He laughed.        Soon the front runners started to disappear, and I saw a woman in a white T­shirt.        “Bob, look！”        We knew we were already watching our “last runner”.She was different from the others because of her ag（2010·上海物理·4）某放射性元素经过11.4天有的原子核发生了衰变，该元素的半衰期为（   ）A．11.4天B．7.6天C．5. 7天D．3.8天