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

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 𝑓𝑠 . The observer hears these sirens at frequency 𝑓𝐴 and 𝑓𝐵, respectively from the two vehicles. Which one of the following is correct? (A) 𝑓𝐴 = 𝑓𝐵 < 𝑓𝑠 (B) 𝑓𝐴 = 𝑓𝐵 > 𝑓𝑠 (C) 𝑓𝐴 > 𝑓𝐵 > 𝑓𝑠 (D) 𝑓𝐴 < 𝑓𝐵 < 𝑓𝑠