A student designs an experiment using a respirometer to compare the rate of aerobic respiration in two different species of small terrestrial invertebrates: woodlice and beetles.

The student outlines the following procedure:

• The chamber is sealed, and the initial position of the coloured liquid droplet in the capillary tube is recorded.
• The respirometer is left for 15 minutes, and the distance moved by the droplet is measured.
• The experiment is repeated with the second species.

Which of the following variables must be controlled to ensure a valid comparison of their respiration rates?

1. The temperature of the chamber throughout the measurement periods.
2. The light intensity, by keeping both setups in complete darkness.
3. The total biomass of the invertebrates inside the chamber.