Which pair does NOT belong to the same classification as the other three?

Three of the pairs join a unit to the quantity it measures — the metre measures length, the litre volume and the kilogram mass. In the odd pair the first word is an instrument, not a unit: a thermometer is the device that reads temperature, whose unit would be the degree. The pair is tempting because the second word is a genuine measurable quantity in every case; the break lies in the first word.