Answer: Option A is not correct because natural gas contains some minor percentage of non-hydrocarbons. Option B is correct regarding the formation of natural gas from small plants and animals in the marine environment. Option C is correct as hydrogen sulfide is known as a very undesirable byproduct of both oil and gas. Hydrogen sulfide is a type of gas that is extremely corrosive, toxic, and flammable and has adverse effects on health, safety, and the environment (HSE). Option D is not correct because methane makes up the highest percentage of natural gas and ethane comes second. Thus, both options B and C are correct.