Martin bikes at a constant speed to a finish line that is 15 mi away. After 20 mins, he is 10 mi from the finish line. After 30 mins, he is 7.5 mi from the finish line. If a graphics shows his distance y from the finish line, in miles, after x minutes, what is the slope of the graph, and what does it represent? A) 15; Martin’s initial distance from the finish line. B) 15; Martin’s final distance from the finish line. C) -0.25; the rate at which Martin’s distance from the finish line changes per minute. D) -0.25; the rate at which Martin’s distance from the start line changes per minute