Answer:
13) a) x axis = Time , y axis = Distance
b) The truck is moving at a constant speed.
Step-by-step explanation:
a) The time is always kept on the x-axis as it is an independent variable.
b) The truck is covering same distance after every equal interval of time. That's why the truck is at a constant speed and the graph is showing no upward or downward moment but fixed at 20 units of y-axis from units 2-6 of x-axis.