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.
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 (which have radiation exposure issues) for concussions. Then it introduces a new blood - test alternative. The new blood - test detects proteins from brain cells in the blood after injury. It not only identifies the presence of head trauma (like the CT scan's purpose of detecting concussions) but also is a better option as it avoids the radiation problem of CT scans. So, it makes detection easier (as it's a simple blood test) and safer (no radiation).
The first option only focuses on the protein - detection aspect, which is part of the blood - test's function but doesn't capture the overall improvement (safer and easier) compared to CT scans. The third option is just about the existing CT - scan method, not the main idea which is the new blood - test as a better alternative.

Answer:

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