The graph shows the distance y, in miles, of a moving train from a station over a certain period of time, x, in hours: what is the speed (in miles per hour) of the train and why? 80 miles per hour, because speed is the initial value of the function 120 miles per hour, because speed is the distance traveled in unit time 240 miles per hour, because speed is the distance traveled in a certain time 40 miles per hour, because speed is the rate of change of distance