Two sources of sound S1 and S2 produce sound waves of same frequency 660 Hz. A listener is moving from source S1 towards S2 with a constant speed u m/s and he hears 10 beats/s. The velocity of sound is 330 m/s. Then u equals:
a. 5.5 m/s
b. 15.0 m/s
c. 2.5 m/s
d. 10.0 m/s