When you drink cold water, your body must expend metabolic energy in order to maintain normal body temperature (37° C) by warmin
g up the water in your stomach. Could drinking ice water, then, substitute for exercise as a way to "burn calories?" Suppose you expend 390 kilocalories during a brisk hour-long walk. How many liters of ice water (0° C) would you have to drink in order to use up 390 kilocalories of metabolic energy? For comparison, the stomach can hold about 1 liter.
I think the correct answer would be D. The tap water in the experiment is one the three test conditions of the independent variable, the type of water. The independent variable in a experiment is the one being manipulated or the one being changed. In this case, it is the type of water.