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

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

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Physics
A cyclist leans inwards when he is negotiating a curve so that

Necessary centripetal force may be available from the horizontal component of the normal reaction due to the track to maintain the curved motion.
He may increase his speed
His weight may be reduced
No frictional force may be produced between the wheels and the axle

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