A、Stop in the middle of the class. B、Speed up the pace. C、March in place for a while. D、Concentrate on exercise. C短文谈到，如果你在有氧课里太

We think of aerobics as being a high impact activity but actually aerobic exercise by definition means we practice the same large muscle group, rhythmically, for a period of 15 to 20 minutes or longer while maintaining 60-80% of your maximum heart rate. Think of aerobic activity as being long in duration yet low in intensity. You should be able to carry on a short conversation while doing aerobic exercise. Aerobic exercise conditions the heart and lungs by increasing the oxygen available to the body and by enabling the heart to use oxygen more efficiently. The aerobic curve occurs when you begin exercising, increase your intensity level, hit the high point and gradually decrease your intensity level. [21] The goal when exercising aerobically is to hit your target heart rate and maintain it for the entire exercise session. This works the heart muscle more effectively and burns more calories. You start, hit your target zone, then you maintain your pace until the cool down. As your heart becomes conditioned, you will have to work harder to reach the target zone. [22]If you are too tired to continue exercising in an aerobic class, march in place for a while until you can resume exercising. It is not OK to stop in the middle of an aerobic class because your body is sending extra blood to the muscles. Stopping suddenly can lead to muscle cramping and dizziness.
19. What do we know about aerobics from the passage?
20. What will happen when we are doing aerobic exercises?
21. What is the goal of aerobics according to the speaker?
22. What does the speaker suggest we do when we are tired in an aerobic class?