The answer for that kind of question depends on many things. location, how many people will inhabit the home, whether or not the home is a vacation home or a full time home. etc. P.S. I do not believe that this has anything to do with chemistry
A: Natural Gas
They do this because Natural gas does not have a smell, so you wouldn't know if it was in the air. So they give it a distinct foul smell, so you know if there is a gas leak.
In the voltage-gated channels, the stimuli is a change in the voltage across the membrane.
In mechanically gated channels the stimuli is a mechanical stress.
In ligand-gated channels the stimuli is the binding of a ligand:ion or molecule that binds to a metal atom.
