A cyclist rides east at 5 m/s along a quiet straight road. Drone P, located east of the cyclist, flies westward directly toward the cyclist at 20 m/s, while Drone Q, located west of the cyclist, flies westward away from the cyclist at 15 m/s. Both drones simultaneously emit identical 900 Hz tones. Take the speed of sound in air to be 340 m/s. What beat frequency does the cyclist hear at that instant?
a) 55 Hz
b) 107 Hz
c) 121 Hz
d) 66 Hz