A submarine (A) travelling at 18km/hr is being chased along the line of its velocity by another submarine
(B) travelling at 27km/hr. B sends a sonar signal of 500Hz to detect A and receives a reflected sound of frequency v. The value of v is close to:
(Speed of sound in water 1500ms⁻¹)

A. 502Hz
B. 499Hz
C. 507Hz
D. 504Hz