You are conducting an experiment to test the hypothesis that dairy cows they will produce more milk if they listen to classical music while being milked. The 20 cows of the experiment listen to classical music during milking; you collect all the milk and measure the quantity. The 20 control cows must have the same conditions experimental cows except that: A. They should listen to classical music at a higher volume. B. They should listen to a different kind of music, like rap. C. They should not listen to music. D. They must not be milked.