a=acceleration depends on the speed and time. if the speed and time are increasing at the same rate, the acceleration value will be positive as the vehicle is speeding up.
b=the speed and time are not increasing, therefore the vehicle is either stationary or travelling at a steady pace.
c=same explanation as a
d=the speed and time are not increasing at the same rate as the speed is decreasing. this means that the car is slowing down
