Physics
The frequency of ultrasound wave is typically

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

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Physics
It is dangerous to observe Solar eclipse with naked eyes because

Cosmic rays reach eyes more during the eclipse
Ultraviolet radiations from Sun burn our retina
Infrared radiations from sun burn our retina
All radiations from sun initiate chemical reactions in eyes

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