[originaltext] The injury to the brain caused by a blow happens when the bra

The injury to the brain caused by a blow happens when the brain is shaken, often in a car crash or a fall or a strike to the head in sports. It can be mild, but doctors may order a CT scan to look for a more serious injury. Computed X-rays provide a detailed image of the brain.(20-1)But a recent study warned that more children than necessary are being exposed to radiation this way. A national team led by two doctors at the University of California, Davis, studied hospital records from thousands of children with head injuries.(20-2)They found that in many cases, the risk of developing cancer from the radiation outweighed the risk of a serious brain injury. The study found that one in five children over age two had a low risk of serious injury but received CT scans anyway. The same was true of almost one in four children under age two. The researchers have developed rules to predict if a head injury is serious enough for a scan.(21-1)For children under two, doctors are advised against it if there is: normal mental activity, no swelling in the back of the head, no feeling of a broken bone in the skull, and no loss of consciousness for more than five seconds. Doctors should also consider how the child was injured and whether the parents say the child is acting normally.(21-2)For patients from two to eighteen, the guidelines are similar—except there should be no loss of consciousness, no vomiting and no severe headache.
Questions 20 to 22 are based on the passage you have just heard.
20. What’s the new discovery about the CT scan?
21. Which of the following statements about a child having a head injury is TRUE?
22. What is the passage mainly about?