article: Everyone knows that exercise is important. We all need to exercise. Doctors say it is good for us. It makes your heart and body strong. Children who often exercise are more alert .
  There are many ways to exercise. You can walk, run, swim, skate, or play ball games. Make sure you exercise in the following ways: you have to like what you're doing. Exercise enough but not too much. It's best to exercise twice each week. Thirty minutes each time is enough. Try all kinds of things until you find one, two or even three sports that feel right for you.
  You can exercise at fitness centers. They have a lot of equipment   there. The equipment will help exercise your arms, legs and other parts of your body to make you fit. Some people buy equipment for their homes. But it is very expensive.
  Exercising can be fun. Friends can exercise together at a fitness center, or they can play s ports together. How do you exercise?
question: _   is enough for exercising.
options: Once a week Twice a week Three times a week Four times a week