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Correct Answer: A
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Sound waves are longitudinal-mechanical wave. Sound waves are divided into three categories that cover different frequency range; Audible wave (20 Htz- 20,000 Htz), Infrasonic waves (<20Htz) and Ultrasonic waves (>20,000 Htz). Ultrasonic waves are used to destroy insects, clean clothes by removing dust, treat diseases, control automatic doors, detection of aircraft and submarine, determination of depth of sea etc.