B

    We all need to exercise. Doctors say it is good for us. It makes our heart and body strong. It also gives you more energy(能量). And you will feel better about yourself. It's best to exercise twice a week. Twenty minutes each time is enough.

    There are many ways to exercise. You can walk, run, play sports or swim. Many people go to special places to exercise. They are called "fitness centers". These places have a lot of equipment(设备). Some people buy equipment for their homes. But it's very expensive.

    Exercising can be fun. Friends can exercise together at fitness centers. Or they can play sports together. How do you exercise? How often do you exercise?

为以下图片填写说明：
   
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帮助别人是快乐的，是有价值的，因为（   ）

有两层书架共有书156本，第二层的书比第一层的2倍还多6本．问第一层有多少本书？