Physics
The Hooke's law is valid for

Elastic as well as plastic region of the stress strain curve
Only proportional region of the stress strain curve
Entire stress strain curve
Entire elastic region of the stress strain curve

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Physics
A hydrogen balloon floats up because of

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

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