article: What's the coolest kind of transportation  for middle school students back from winter holidays? A racing bike? A car? No, it's a special kind of shoes called Heelys . Heelys look just like common sports shoes, but they have a wheel  hidden  in the heel . So instead of walking, kids can "fly" around in them.
  "Wearing Heelys is fun and cool!" said Wu Peng, a boy who wore them on his first day back at No. 6 Middle School in Beijing. Wu Peng said he loves the shoes so much that he wears them to go here and there. Sometimes he even follows his parents' car to the supermarket in his Heelys!
  Other students also think they are very cool, but some aren't so lucky with their Heelys. It's said that some children fell down and got hurt while wearing these shoes.
  "Heelys wheels are in the heels of the shoes, so it's easy to fall," said Liu Rui, a doctor at the Hong Kong International Medical Clinic, Beijing. Even worse, Liu said, "Wearing Heelys for a long time could stop young people from developing their legs. "
question: What does the doctor think of Heelys?
options: They are too expensive to buy. They are too fast to go. They are bad for kids' health. They are good for training.