Answer: 3 4 2 1
Notes: Compressed natural gas (CNG) is made by compressing natural gas which is composed of methane (CH4), it also contain small amount of ethane.
Coal gas typically contains hydrogen, methane and carbon monoxide.
LPG is the abbreviation or short form of Liquefied Petroleum gas. The major constituents of LPG is propane and butane.
Water gas is a synthesis gas, containing carbon monoxide and hydrogen.