An engine approaches a hill with a constant speed. When it is at a distance of 0.9 km, it blows a whistle whose echo is heard by the driver after 5 seconds. If the speed of sound in air is 330 m/s, then the speed of the engine is :
A. `10 m//s`
B. `20 m//s`
C. `30 m//s`
D. `40 m//s`