Physics
Hydroscope is an instrument that shows changes in:

None of these
Density of liquid
Sound under water
Atmospheric humidity

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Physics
Planets are

non-luminous heavenly bodies
luminous heavenly bodies that do not twinkle
luminous heavenly bodies that twinkle
luminous heavenly bodies revolving around a star

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Physics
Railway tracks are banked on curves

necessary centrifugal force may be obtained from the horizontal component weight of the train
necessary centripetal force may be obtained from the horizontal component of the weight of the train
the train may not fly off in the opposite direction
to avoid frictional force between the tracks and wheels

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