A jet plane in straight horizontal flight passes over your head. When it is directly above you, the sound seems to come from a point behind the plane in a direction 30◦ from the vertical. The speed of the plane is:
A. the same as the speed of sound
B. half the speed of sound
C. three-fifths the speed of sound
D. 0.866 times the speed of sound
E. twice the speed of sound
