answer: The doctor asks #Person1# to take an X-ray and the doctor puts #Person1# in a cast because the X-ray picture shows that #Person1#'s humerus is broken.
documents: #Person1#: Doctor, I slipped and fell on my way to school. The ground is so slippery from the snow. It seems I can't move my left arm now.
  #Person2#: Let me see. Roll up your sleeve, please. Um. . . it's swollen and red here. I'm afraid we'll need to take an X-ray to see if it's broken.
  #Person1#: What if it's broken?
  #Person2#: Then we'll put you in a cast.
  #Person1#: What do you mean by a cast?
  #Person2#: Well, we apply tape and plaster to from a solid enclosure to protect the bones from moving. In this way they will heal properly. Take it easy. It won't hurt you. Let me see. The X-ray picture indicates that your humerus is broken in two places. And you'll have to wear the cast for three weeks. You'll need to come back in two weeks, so I can have another look.