Generally, natural gas consists of ... gas

Instruction: You are offered the test items on the base of context with one correct answer from five proposed ones. Read the context attentively and do the items.

Natural Gas
Natural gas is a fuel that's used to heat buildings, cook food, dry clothes, heat water, and even to produce electricity. It's sometimes called "gas" for short, but don't confuse it with the gasoline that runs your car. Gasoline is a liquid, while natural gas is a gas!
In fact, natural gas is really a mixture of gases that formed from the fossil remains of ancient plants and animals buried deep in the earth. The main ingredient of natural gas is methane.
Methane is odorless and colorless gas. So, why does natural gas smell? The gas companies add a chemical called mercaptan, which gives natural gas that funny sulfur-like or rotten egg odour.
Natural gas release a lot of heat and light when it burns, but doesn't produce smoke. That makes it a good fuel to use in houses. Today a lot of houses are heated by natural gas.
Natural gas is a popular fuel choice because it burns cleaner, hotter, and brighter than other fossil fuels like coal and oil. It's also safe. Because natural gas pipes are buried safely underground.