There is no scientific consensus on why we yawn.
One theory argues that when humans are tired, we stop taking deep breaths, which causes a build-up of carbon dioxide in the body, said Douglas Parham, a speech scientist at Wichita State University. A yawn might be a special kind of respiration, Parham said. It builds up oxygen quickly, while expelling carbon dioxide, even more than a deep breath.
Excess carbon dioxide and other chemical changes, such as a drop in oxygen or increase in a compound called adenosine, could also act as “yawn gates,” said James Giordano, a neuroethicist and neuroscientist at Georgetown University. These chemicals send out a signal that triggers a yawn. By yawning, we compress the muscles of the face, driving oxygen-enriched blood to the brain, Giordano said.
Other proposals state the purpose of yawning is to cool the brain, or to stretch internal organs like tissues and lungs, helping the body to liven up.
Question
Use the excerpt to answer the question.
Which matter is left uncertain in this excerpt?
(1 point)
Responses
the idea that yawns involve the muscles of the face
the idea that yawns involve the muscles of the face
the chemicals that make up “yawn gates”
the chemicals that make up “yawn gates”
the exact scientific reason for yawning
the exact scientific reason for yawning
the reason that yawns are a special kind of respiration
the reason that yawns are a special kind of respiration