Mortimer Poindexter spends two hours every day running away from bullies. Some of the bullies he encounters are faster than others; therefore, he sometimes needs to run faster than at other times, but he is always chased for exactly two hours. (These bullies stick to a very rigid schedule.) Let s be the speed Mortimer runs (in miles per hour) and let d be the distance he travels (in miles) as he tries to make his escape. Is s a constant or a variable?