Many are familiar with ticker-tape timer. The tape is attached to Car B. As Car B moves to the left, it pulls the tape with it. The tape will go through the ticker-tape timer machine. The machine is stationary and it just punches holes on the tape as the tape is pulled to the left. It is clear that Car A is accelerating as the distance interval of the holes is getting wider.
For Car B, the scenario is different. Oil is leaking from the car onto the road at fixed interval or frequency. Hence Car B is decelerating as the distance interval of oil drops becomes smaller.
Therefore, be careful with the two scenarios and do not be confused.