Physics
A hydrogen balloon floats up because of

Air pressure decreases with decrease in height
Weight of the balloon is less than the weight of air displaced by it.
Air pressure decreases with decrease in weight
The pressure inside the balloon is more than the pressure outside it

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Physics
Stationary wave is formed by

A transverse wave superposing a longitudinal wave
Two waves of same frequency travelling in the same direction
Two waves of same frequency travelling in the opposite direction
Two waves of the same speed superposing

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