Achemical formula written above or below the yield sign indicates:
a. that a gas is formed.
b. that the substance is used as a catalyst.
c. that heat must be supplied.
d. a reversible reaction.

I believe the correct answer from the choices listed above is option B. A chemical formula written above or below the yield sign indicates that the substance is used as a catalyst. I am certain with this answer. Hope this helps. Have a nice day.