Which line from the text shows why scientists continue to study yawning?

one thing is certain: you are probably fighting the urge to yawn right about now.
other scientists believe, however, that yawning is a form of communication.
one mystery that still remains unsolved is why we yawn—what makes us yawn and what purpose it serves.
the theory is that yawning cool the brain, allowing it to work more efficiently.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.