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A boy is walking away from a wall toward...

A boy is walking away from a wall towards an observer at a speed of 1 metre / sec and blows a whistle whose frequency is 680 Hz . The number of beats heard by the observer per second is (Velocity of sound in air = 340 metres / sec

A

Zero

B

`2`

C

`8`

D

`4`

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The correct Answer is:
D
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