Physics
The bats can fly in the dark because

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

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Physics
Two identical solid pieces, one of gold and other of silver, when immersed completely in water exhibit equal weights. When weighed in air (given that density of gold is greater than that of silver)

The silver piece will weigh more
The gold piece will weigh more
Both silver and gold pieces weigh equal
Weighing will depend on their masses

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