Physics
Why do birds not have respiratory trouble at the time of flying at high altitude ?

They fly inactively
Their lungs are very large
They use less oxygen
They have extra air sacs

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Physics
It is more difficult to walk on a sandy road than on a concrete road because

the sand is grainy but concrete is smooth.
sand is soft and concreter is hard.
the friction between sand and feet is more than that between concrete and feet.
the friction between sand and feet is less than that between concrete and feet.

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