Question

informational text evidence quick check
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
use the excerpt to answer the question.
which matter is left uncertain in this excerpt?
(1 point)
the reason that yawns are a special kind of respiration
the chemicals that make up yawn gates
the idea that yawns involve the muscles of the face
the exact scientific reason for yawning

Explanation:

The text starts by stating there is no scientific consensus on why we yawn and then presents various theories. This indicates that the exact scientific reason for yawning remains uncertain.

Answer:

the exact scientific reason for yawning