Which gas is NOT primarily composed of hydrogen sulfide and oxygen?

Methane is considered to be the correct choice because it is primarily composed of carbon and hydrogen, with the chemical formula CH4. This means that it does not contain significant amounts of hydrogen sulfide (H2S) or oxygen (O2) in its composition.

On the other hand, hydrogen sulfide is directly named in the question and is composed of hydrogen and sulfur, while oxygen is a standalone gas essential for combustion and respiration.

Carbon dioxide consists of carbon and oxygen (with the chemical formula CO2) and is also unrelated to hydrogen sulfide. Chlorine is a diatomic molecule composed of two chlorine atoms (Cl2) and is distinct from both hydrogen sulfide and oxygen as it relates to its toxicity and reactivity.

Therefore, among the options, methane stands out as the gas that does not share the characteristic composition involving hydrogen sulfide and oxygen.