阅读下列短文, 在每小题所给的A、B、C、D四个选项中选出最佳选项。
You might think that Olympic athletes are the healthiest people in the world. It's true that many are. However, it's also true that many Olympic athletes had to overcome (克服) illness early in their lives.
One excellent example is Wilma Rudolph. She competed in track-and-field (田径) events in the 1960s Olympics. She didn't win just one gold medal (奖章).She won three. At the time, people called her ”the fastest woman in the world”.
Actually, as a young child, Wilma could not take part in sports. She had serious illnesses, and then, at the age of 4, she got polio (小儿麻痹症).She lost the function of her left leg, and the doctor said she could never walk again.
Wilma's family did everything they could to help her walk again. Wilma and her mother used to travel 100 miles to get treatments for her leg. Her brothers and sisters took turns to give her leg a daily massage (按摩).Four times a day, they helped her do special exercises for her leg. Amazingly, by the time Wilma was 9 years old, she was able to walk again. Before long, she started playing basketball and running. In high school, she was a track star and then she went to the Olympics.
Wilma stopped being a runner when she was 22 years old. She then became a teacher and a track coach. Her story encouraged many people to work hard and to overcome difficulties.
治疗肿瘤，该核素原子核内的中子数与电子数之差是