题目

 One of the easiest ways to keep healthy is to do jogging. “ Jogging is the name for a very gentle （温和的）running ——it is just a little faster than walking.”     Start slow jog 20 meters, then walk 20 meters. Little by little, if you are not feeling very tired, you may do more jogging and less walking. Finally, jog the whole day. It will be easy for you to start jogging for 15 minutes twice a week. Then increase(增加) this to 20-30 minutes every day. The longer you join in jogging, the more you enjoy it and the healthier you will become. Some people like jogging a在化学反应前后肯定不变的是（ ）A．物质的种类B．物质的性质C．分子的种类D．原子的种类