Questions

A greenhouse gas is a gas in an atmosphere that absorbs and emits radiant energy within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. 

Carbon dioxide is a main greenhouse gas, found in car exhaust pipes.
Methane and nitrous oxide is another type of greenhouse gas.

Oxygen is the only clean gas out there because we need it to survive. Therefore, the only gas that is not a greenhouse gas is Oxygen.

Steel is not a compound or element. Steel is an alloy of iron and carbon and other elements and hence it is a mixture. Because of its high tensile strength and low cost, it is a major component used in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons.

Homer's literature had created the foundation for literature and was influential in his time and the future. And they are credited as foundational components of the Western worldview.

Yes, future is always infront of us, but no one can see it.