Three of the four instruments named below measure something about the weather and one does not. Choose the odd one out: Anemometer, Hygrometer, Rain gauge, Lactometer.

An anemometer measures wind speed, a hygrometer measures the moisture held in the air and a rain gauge measures the rain collected — all three belong to a weather station. A lactometer measures the density of milk to test its purity, so it alone stands outside the group.