Question

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 exact scientific reason for yawning ∘ the idea that yawns involve the muscles of the face ∘ the reason that yawns are a special kind of respiration ∘ the chemicals that make up \yawn gates\

Explanation:

1. Analyze Option 1: The excerpt starts with "There is no scientific consensus on why we yawn" and then presents multiple theories (respiration - related, chemical triggers, cooling brain, stretching organs). So the exact scientific reason for yawning is uncertain as there's no consensus and multiple theories exist.
2. Analyze Option 2: The excerpt says "By yawning, we compress the muscles of the face...", so the idea that yawns involve face muscles is stated, not uncertain.
3. Analyze Option 3: The excerpt explains that a yawn "might be a special kind of respiration" and gives the reason (builds up oxygen, expels CO₂ more than a deep breath), so this reason is provided, not uncertain.
4. Analyze Option 4: The excerpt lists "Excess carbon dioxide and other chemical changes, such as a drop in oxygen or increase in a compound called adenosine" as "yawn gates", so the chemicals are identified, not uncertain.

Answer:

the exact scientific reason for yawning