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Ultrasonic differs from supersonic because supersonic refers to

AFrequency less than 20 Hz only
BFrequency greater than 20 kHz only
CSpeed less than the speed of light only
DSpeed greater than the speed of sound in air
Answer & Solution
Correct answer: D. Speed greater than the speed of sound in air
1. The chapter explicitly distinguishes ultrasonic from supersonic. 2. Ultrasonic = frequency above 20 kHz. 3. Supersonic = speed greater than that of sound in air. 4. So option A is correct. _Source: Selina Concise Physics Class 9, Ch 8 Sound (Mistral OCR'd PDF), section on supersonic vs ultrasonic_
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