The speed of a boat in still water is 21 km/hr and the rate of current is 8 km/hr. The distance travelled downstream in 13 minutes is	Select the correct option:
A) 1.6 km
B) 2 km
C) 6.3 km
D) 4 km
E) none of these

Rationale: "explanation : speed downstreams = ( 21 + 8 ) kmph = 18 kmph . distance travelled = ( 29 x 13 / 60 ) km = 6.3 km option c"
Answer: C