A source of sound and listener are approaching each other with a speed of $40 m/s.$ The apparent frequency of note produced by the source is $400 \,cps$. Then, its true frequency (in $cps$) is (velocity of sound in air $= 360 m/s$)
  • A$420$
  • B$360$
  • C$400$
  • D$320$
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