A engine is approaching a hill at 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 s`. If the speed of sound is `340 m//s`, calculate the speed of the engine.
A. `10 m//s`
B. `20 m//s`
C. `30 m//s`
D. `40 m//s`