Doppler Effect and Beats

2000 AIPMT 1 question

1. 1 Waves
   Two stationary sources each emitting waves of wavelength $\lambda$. An observer moves from one source to other with velocity u. Then number of beats heard by him :
   A $\dfrac{2u}{\lambda}$ B $\dfrac{u}{\lambda}$ C $u\lambda$ D $\dfrac{u}{2\lambda}$
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