A car travels the first one third of a certain distance with a speed of 10 km/hr, the next one third distance with a speed of 20 km/hr and the last one-third distance with a speed of 60 km/hr. The average speed of the car for whole journey is

• A. 18 km/hr
• B. 24 km/hr
• C. 30 km/hr
• D. 36 km/hr