A Bat Emits a Sound at a Frequency of 39

Multiple Choice

A bat emits a sound at a frequency of 39.0 kHz as it approaches a wall. The bat detects beats with a frequency of 827 Hz between the sound it emits and the echo bouncing from the wall. What is the speed of the bat? The speed of sound in air is 343 m/s.

A) 2.7 m/s
B) 3.0 m/s
C) 3.6 m/s
D) 5.4 m/s
E) 9.0 m/s