C is the correct answer. It is healthy to get exercise everyday, but if you push yourself too hard, you can harm yourself. For example, lifting weights that are much heavier than you can hold, is dangerous because the stress that you are feeling while lifting could have damage to bone structure in your arms or legs, or even cause a blown blood vessel in the brain. So I believe C is the correct answer because over-training may cause you to get hurt.
A. Not the best answer choice
B. False, proper training each day can help you get good at a sport
C. Correct answer
D. False, many young athletes in high school sports have had severe injuries due to over-training.