Physics
The bats can fly in the dark because

They generate ultrasonic sound waves
They have weak legs and are likely to be attacked by predators
They generate flashes of light
They can see the objects in darkness

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Physics
What is refractive index ?

None of above
It is defined as the product of speed of light in medium and in vacuum.
It is defined as the ratio of speed of light in the medium to the speed of light in vacuum.
It is defined as the ratio of speed of light in vacuum to the speed of light in the medium.

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