For speed above 3000 km/hour, it is more advantageous to use____________________?

(A) turbo-jet engine (X)
(B) ram-jet engine ()
(C) propellers (X)
(D) rockets (X)
(E) hydraulic jet propulsion (X)

Answers: For speed above 3000 km/hour, it is more advantageous to use  ram-jet engine.