We all need to exercise. Doctors say it is good for us. It makes your heart and body strong. It also gives you more energy. And you will feel better 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 equitment. Some people buy equitment for their homes. But it is very expensive.Exercising can be fun. Friends can exercise together at fithess center. Or they can play sports together, how do you exercise?

4个9相加，乘法算式为____。

位于我国东北的重工业基地是（   ）