A motorist wishes to travel 40 kilometers at an average speed of 40 km/h. During the first 20 kilometers, an average speed of 40 km/h is maintained. During the next 10 kilometers. however, the motorist averages only 20 km/h. To drive the remaining 10 kilometers and average 40 km/h, the motorist must drive
a) 60 km/h. b) 80 km/h. c) 90 km/h. d) faster than the speed of light