Water learned in science class that different substances release heat at different rates. he decides to test this. at home, walter turns the oven to 200°f. he places three casserole dishes in it: one is made of metal, one is made of glass, and one is made of ceramic. he leaves them in the oven for an hour. he then removes them from the oven, places a single ice cube on each one, and times how long it takes the ice cube to melt. in this experiment, what is the test variable?