Two vehicles A and B are approaching an observer O at rest with equal speed as shown in the
figure. Both vehicles have identical sirens blowing at a frequency ππ
. The observer hears these
sirens at frequency ππ΄ and ππ΅, respectively from the two vehicles. Which one of the following is
correct?
(A) ππ΄ = ππ΅ < ππ
(B) ππ΄ = ππ΅ > ππ
(C) ππ΄ > ππ΅ > ππ
(D) ππ΄ < ππ΅ < ππ