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A locomotive approaching a crossing at a...

A locomotive approaching a crossing at a speed of 20 `ms^(-1)` sounds a whistle of frequency 640 Hz when 1 km from the crossing. There is no wind and the speed of sound in air is 330 `ms^(-1)`. What frequency is heard by an observer `sqrt(3)` km on the straight road from the crossing at right angle :-

A

600 Hz

B

630 Hz

C

660 Hz

D

720 Hz

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The correct Answer is:
C
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