Question

The graph shows the distance, y, in miles, of a moving train from a station over a certain period of time, x, in hours:

http://prnt.sc/dkb8cj

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

I think its the last one because it's going up by 40