A toy car has an initial acceleration of 2m/s^2 across a horizontal surface after it is released from rest. After the car travels for a time t consisting of only the car an open system or a closed system, and why?

a. Open system because the acceleration of the car is not constant.
b. Open system, because an external force is applied to the car that causes it to accelerate.
c. Closed system, because the speed of the car is as expected in the case where an object has uniform acceleration for a timet
d. Closed system because mechanical energy was not removed from the system as a result of a net force