17.
Consider the following statements about ultrasonic waves.
1. They can destroy insects.
2. They can clean clothes by removing dust.
3. They can be used to treat diseases.
4. They can control automatic doors.
Of the above statements.

19.
Which of the following is/are true regarding speed of sound in different mediums at 25°C?
A. In aluminium, the speed of sound is 6220m/s.
B. In nickel, the speed of sound is 6040 m/s.

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