Two sound sources emitting sound each of wavelength $\lambda$, are fixed at a given distance apart. A listener moves with a velocity v_o along the line joining the two sources. The number of beats heard by him per second is

• A

  ${\mathrm{v}}_{\mathrm{o}}/2\mathrm{\lambda }$

• B

  $2{\mathrm{v}}_{\mathrm{o}}/\mathrm{\lambda }$

• C

  ${\mathrm{v}}_{\mathrm{o}}/\mathrm{\lambda }$

• D

  ${\mathrm{v}}_{\mathrm{o}}/3\mathrm{\lambda }$