Three pairs below are matched on one principle of classification; one is matched on a different principle. Choose that pair.

Three of the pairs join a gas to the work it is put to — helium lifts balloons because it is lighter than air, chlorine is added to water to kill the germs in it, and neon glows red in a sealed tube to make a sign. The odd pair gives a property of the gas instead of a use: being colourless is what oxygen is like, not what it is for, and its use would have been the support of breathing and burning.