Two sources \[{{S}_{1}}\] and \[{{S}_{2}}\] of same frequency\[f\]emits sound. The sources are moving as shown with speed \[u\] each. A stationary observer hears that sound. The beat frequency is (\[v\] = velocity of sound)

• A. \[\frac{2{{u}^{2}}f}{{{v}^{2}}-{{u}^{2}}}\]          
• B. \[\frac{2{{v}^{2}}f}{{{v}^{2}}-{{u}^{2}}}\]
• C. \[\frac{2\,u\,vf}{{{v}^{2}}-{{u}^{2}}}\]        
• D. \[\frac{2u}{v}f\]