16. A car travels 40 kilometers at an average speed of 80 km/hr and then travels another40 kilometers at a speed of 40 km/hr. The average speed for the trip is a) 40km/hr b) 45km/hr c) 48 km/hr d) 53 km/hr e) 73 km/hr 17. When the brakes of an automobile are applied, the road exerts the greatest retarding force on the car a) while the tires are sliding b) just before the tires start to slide c) When the car is going fastest d) when the acceleration if the least e) at the instant the speed begins to change