Dr. Trout has noticed that salmon in his fish hatchery seem to enjoy listening to the radio in his lab. The fish in the tank closest to the radio seem to grow larger. Dr. Trout decides that the more music he plays, the larger the fish must grow. To test his idea, he sets up 5 large ponds with 100 salmon in each pond. He places a loudspeaker at the middle of each pond and plays various amounts of music in each pond. He plays the same band, feeds the fish the same diet, and keeps water conditions in all tanks exactly the same. After 10 weeks, he ends the trial and takes the average mass of the fish in each pond, and presents his data in the table. The independent variable in Dr. Trout's experiment is 
A) type of fish. 
B) the fish diet. 
C) type of music played. 
D) hours of music played.