阅读下列短文，然后根据短文的内容从每小题的四个选择项中选出最佳的一项。

     Nowadays everyone is so much trying to keep fit that sometimes they miss the fact there are times when they should not be exercising at all. Here you can find real reasons to stop training for a while and do not feel sorry for your laziness.
     When you are ill
     Do not exercise if you are not feeling well, even if it is just a cold. Workout can hurt your immune system (系统) and lengthen illness. As a result, you will spend more time away from the physical activity. Training while being ill is also a main cause of other injuries since it is more difficult to focus on (专心于) what you are doing when you feel bad.
     When you have not had enough recovering time
     Do not rush back to your normal workout habit after you have been ill. Starting to train too early is likely to lead to a return of your symptoms (症状) . When you do decide to go back to the sports ground, be sure to start your exercise slowly and with care. Even one week or so off your usual exercise can make a difference.
     When you are tired out
     There sometimes are days when you do not feel like going to the sports ground, and sometimes your body might be telling you to take a break. It will do you a lot of good to have a day off from your fitness classes. Go home, have a healthy meal and do not feel sorry for not being at the sports ground.
     Remember that sometimes you just need to relax.