An experiment was designed to invesitgate the effect of caffeine on the heartbeat of water fleas. Two populations of water fleas were cultured. Both populations had water with the same mineral content, were supplied with identical amounts of bacteria as food, received the same amount of light, and had their temperature meainted at 20ºC. Every two hours water fleas from both populations were selected and their heartbeats monitored. The fleas of population one had caffeine amdnistered five minutes before the heartbeat was checked. The fleas of population two were given nothing. Match each factor with the correct term from the list below. The terms may be used more than once.