A Concussion（脑震荡） happens when the brain is shaken, often in a car crash or a fall or a strike on; the head in sports．Concussions can be mild, but doctors may order a CT scan to look for a more serious injury．But a recent study warned that more children than necessary are being exposed（使遭受） 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. 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 two years of age.
The researchers have developed rules to predict if a head injury is serious enough for a scan. 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. (头骨)
·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.
For patients from two to eighteen, the guidelines are similar —— except there should be no l vomiting（呕吐）and no severe headache.
Earlier this year, the British Journal of Sports Medicine published new guidelines for concussions in children and teens. International experts said they should not return to sports or school until fully recovered. The brain also needs a "cognitive rest," they say, by restricting activities like video games, texting and watching TV.
It often take longer than adults to recover from a concussion than adults. The experts say individual progress and not a set time period should always guide a decision to return to play.
The researchers at the -University of California found that children who received CT scans

A．were likely to suffer brain injuries

B．wouldn’t have normal mental activity

C．would lose consciousness now and then

D．were likely to develop canter

How many aspects should doctors consider when deciding whether a child under two needs a CT scan?        

A．Four．

B．Six．

C．Seven．

D．Nine．

What can we learn from the last two paragraphs?

A．Texting is harmful to patients’ recovery from concussions．

B．Patients with brain injuries can play many sports．

C．Adults need a longer time than teens to recover from concussions．

D．It takes at least a year for individuals with brain injuries to fully recover．

The author of the passage mainly___．

A．describes the risks of brain injuries

B．suggests CT scans are of practical use

C．tells us about the risks of brain injury tests

D．argues against new guidelines for concussions