Physics
A balloon filled with hydrogen will

Reach a particular height and start coming down
Burst after reaching some height
Continue going upwards uninterrupted
Reach a particular height and remain floating

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Physics
Stationary wave is formed by

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

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