A man covers half part of a certain distance at 15 km/hr. He covers 40 percent of remaining distance at the speed of 12 km/hr and covers rest of the distance at the speed of 21 km/hr. What is his average speed for the entire journey?

A

140/9 km/hr

B

15 km/hr

C

164/9 km/hr

D

139/9 km/hr