Question

read the passage.
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.
doctors commonly use ct scans to test for brain injuries.
a new blood test makes it easier—and safer—to detect concussions.

Explanation:

The passage first introduces CT scans as a common concussion test, notes their radiation-related drawbacks, then presents a new blood test as a safer alternative that can diagnose head injuries. The central idea ties the limitation of the existing method to the benefit of the new test. The first option only describes a detail of the blood test, the second only covers the CT scan part, while the third captures the full core: the new blood test is a better, safer way to detect concussions.

Answer:

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