Physics
Hydroscope is an instrument that shows changes in:

None of these
Atmospheric humidity
Sound under water
Density of liquid

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Physics
Light houses are places with powerful lights to :

Indicate to the incoming warships, the location of a harbour during night
Guide and resolve traffic jams in crowded metro-cities during nights
Guide and help large crowds at religious gathering during nights
Guide and warn the ships coming from different directions in the ocean

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