A car is moving at 10 m/s to the right. It accelerates for 10 s after which it is moving at 5 m/s to the left. What was the car's acceleration during the 10 s interval? (a)-1.5 m/s^2 (b) -0.5 m/s^2 (c) 9.8 m/s^2 (d) 0.5 m/s^2 (e) 1.5 m/s^2