Question 18 is based on the passage you have just heard.

    Good transportation is very important in winter. (16) If you have a car, make sure it is ready for the cold weather. Keep the gas tank as nearly full as you can. This will keep water out of the tank and will be a reserve in case you get into trouble. (17) If a storm traps you in your car, there are some steps you should take for your own safety. Do not tend to walk to find help. You may quickly lose your way in blowing and drifting snow. Your chances of being found are better if you stay in your car. Keep a downwind window open slightly for fresh air. Freezing rain can seal off your car and lock you inside. Run the engine and heater once in a while. Keep the same downwind window open while the engine is running. Make sure that snow has not blocked the exhaust pipe. Clap your hands and move your arms and legs from time to time. Do not stay in one position too long. But, do not move too much. (18) Exercise warms you up, but it also causes you to lose body heat. If more than one person is in the car, do not sleep at the same time. One person should always be awake. If you are alone, stay awake as long as you can. Turn on the inside light at night. This would make your car more visible to rescue crews. Don’t panic. Stay with your car.

(18). Why is too much exercise undesirable when you are trapped in a car by a winter storm?

解析：根据文章中“Exercise warms you up, but it also causes you to lose body heat.”可知，运动会让你身体暖和起来，但做太多运动会散失身体热量，因此选择C项。