more psychology

we're studying for our first exam and taking it very seriously. until we came across this question:
a researcher wants to see whether a protein-enriched diet will enhance the maze running performance of rats. One group of rats is fed the high protein diet for the duration of the study; the other group continues to receive ordinary rat chow. In this experiment, the diet fed to the two groups of rats is the ______ variable?
CHOW?


Yellow Rat Bastard. I think its a magazine...