A motorist wishes to travel 40 kilometers at an average speed of 40 km/hr. During the first 20 kilometers, an average speed of 40 km/hr is maintained. During the next 10 kilometers, however, the motorist goofs off and averages only 20 km/hr. To drive the last 10 kilometers and average 40 km/hr, the motorist must drive

 

    

 

    (a) 60 km/hr
    (b) 80 km/hr
    (c) 90 km/hr
    (d) faster than the speed of light