Why would using a genetically modified organism (or custom-made organism) such as recombinant bacteria to study how a virus that affects animals could be spread to humans would be controversial.?

a. decomposes use carbon from dead organisms as fuel for cellular respiration, returning carbon dioxide back into the atmosphere.

the answer to your question is tooth decay.

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