Passage Two
We think of aerobics as being a high impact activity but actually [29]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. [30]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. [31]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. [32]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.

Questions 29 to 32 are based on the passage you have just heard.
29. What do we know about aerobics from the passage?
正确答案：B
解析：文章开篇就介绍了什么是有氧运动，B与此一致。
30. What will happen when we are doing aerobic exercises?
正确答案：B
解析：短文谈到，在做有氧运动时你应该能够进行短时间的谈话，故选B。
31. What is the goal of aerobics according to the speaker?
正确答案：B
解析：短文谈到，有氧运动的目标就是在运动的全过程保持目标心跳率，故选B。
32. What does the speaker suggest we do when we are tired in an aerobic class?
正确答案：C
解析：短文谈到，如果你在有氧课里太累了以致于无法继续锻炼，那就原地踏步一会儿直到你可以恢复锻炼为止，故选C。