A 2000 kg car is pulling a 1000 kg trailer. The car's engine exerts 6000 N of force to move the car and the trailer. In addition, the car and the trailer experience a 1000 N frictional force as they are being pulled. What is the magnitude of the net force on this system ?

a. 2000 N
b. 4000 N
c. 5000 N
d. 8000 N