Questions

The olfactory system is the only system in which the receptors send their signal directly to the higher brain and bypass the thalamus, which normally acts as a filter for the processes of the lower brain.

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.

Primary key helps us to identify a row uniquely in a relation

A Skeleton is what is whole but incomplete, have no eyes, yet can see. We can see, and see right through it. The largest part is one fourth of what it once was i.e body.