Chemical mechanisms that can turn off or reduce an enzyme are . a.) inhibitors
b.) activators
c.) centrioles
d.) vacuoles

Explanation:

Chemical mechanisms that can turn off or reduce an enzyme are inhibitors. Explanation; -Enzyme inhibitors are molecules or compounds that bind to enzymes and result in a decrease in their activity. They can bind to an enzyme and stop a substrate from entering the enzyme's active site and/or prevent the enzyme from catalyzing a chemical reaction.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds

when it is at an intermediate population density