Turtles are cold blooded, meaning they use the Sun to keep warm. A student hypothesizes that his pet turtle will prefer swimming in warm water. He places his turtle on a rock in the middle of the bathtub, and measures how much time the turtle spends off of the rock during a five minute period. He conducts the experiment under four conditions in the bathtub: no water, warm water, cool water , and room temperature water. In this experiment, which is the control group ?