Two Trumpet Players Are Both Playing a Pitch with a Frequency

Multiple Choice

Two trumpet players are both playing a pitch with a frequency of 440 Hz,corresponding to the musical pitch A above middle C.However,one of the trumpet players is marching away from you so that you hear a beat frequency of 4 Hz from the two trumpets.With what speed is the departing trumpet player moving away from you? (The speed of sound in air is 340 m/s)

A) 3.12 m/s
B) 3.09 m/s
C) 3.06 m/s
D) 3.00 m/s
E) 2.95 m/s