Question

testing for concussions
when a patient is suspected of having a concussion, or brain injury, doctors often turn to computed tomography, or ct, scans to find out for sure. in a ct scan, a series of x - rays is taken. each one shows an individual \slice\ of the brain, and they can be combined to form a complete three - dimensional picture. because of the amount of radiation patients are exposed to, however, doctors question the use of the scans, especially for children. now researchers have proposed an alternative way of diagnosing head injuries: a simple blood test. the test detects proteins that burst out of brain cells into the bloodstream immediately after an injury. it allows doctors to determine whether a patient has suffered head trauma and needs further treatment.
what is the main, or central, idea of the passage?
a new blood test can identify proteins that are present in the blood following a brain injury.
a new blood test makes it easier - and safer - to detect concussions.
doctors commonly use ct scans to test for brain injuries.

Explanation:

The passage first mentions the use of CT - scans for diagnosing concussions and their drawbacks due to radiation exposure. Then it focuses on a new proposed blood - test as an alternative for detecting head injuries. The main idea is about the new blood - test making detection of concussions easier and safer compared to CT - scans. The first option only focuses on the blood - test identifying proteins and the third option only mentions CT - scans.

Answer:

A new blood test makes it easier—and safer—to detect concussions.