题目

Cyril was a small town. The houses there were all made of wood; almost everything in the houses, such as cupboards, tables and chairs were made of wood, too. And all the houses faced south. The town was never peaceful. Fire broke out once a week on summer afternoons. People there were puzzled who the real firebug (放火者) was. One day a big fire broke out. It was said that a Negro had jumped out of the house on fire, so the police arrested all the Negroes in the street. However, fire never stopped to break out when the Negroes were in prison. Watson, a physics teacher didn't believe下列说法中，正确的是（ ）A．在与外界隔绝的条件下，物质间不论发生何种变化，其总质量保持不变B．aL氢气和bL氧气完全反应，生成（a+b）L水C．在化学反应前后，生成物的质量总和一定等于参加反应的各物质的质量总和D．引燃密闭容器中的ag氢气和bg氧气，冷却后得到水的质量一定等于（a+b）g