A biologist wants to know how the absence of a certain mineral nutrient will affect the bone growth of a laboratory rat. The biologist performs the following four experiments. Experiment 1: Feeds the rat 100 mg of the nutrient a day, and measures bone growth over 30 days. Experiment 2: Feeds the rat 50 mg of the nutrient a day, and measures bone growth over 30 days. Experiment 3: Feeds the rat 25 mg of the nutrient a day, and measures bone growth over 30 days. Experiment 4: Feeds the rat 0 mg of the nutrient a day, and measures bone growth over 30 days. Which is the control experiment?
A. E1
B.E2
C.E3
D.E4
E= Expirement