Physics
The frequency of ultrasound wave is typically

Below 02 kHz
Below 20 kHz
Above 20,000 kHz
Above 20 kHz

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Physics
A hydrogen balloon floats up because of

Weight of the balloon is less than the weight of air displaced by it.
The pressure inside the balloon is more than the pressure outside it
Air pressure decreases with decrease in height
Air pressure decreases with decrease in weight

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