Answer:
B
Explanation:
It is not A because if they were riding for a constant speed the entire ride it would just be a straight line going up and that line is curved.
It is B because in the chart you see that from hour 3 to hour 4 they did not move at all (this is why it is a flat horisontal line) and you can infer that they stopped and took a break.
It is not C because they stopped and took a break at hour 3 to hour 4.
It is not D because they never decreased their milage.