Physics
In radio-communication, the signals emitted by transmitting antenna are reflected on

Ozonosphere
Troposphere
Stratosphere
Ionosphere

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Physics
On a clean glass plate a drop of water spreads to form a thin layer whereas a drop of mercury remains almost spherical because:

mercury is a metal.
cohesion of mercury is greater than its adhesion with glass.
density of mercury is greater than that of water.
cohesion of water is greater than its adhesion with glass.

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Physics
Skating on ice is possible because

The surface of ice is smooth
The pressure on the ice due to skate melts the ice by lowering its melting point forming a thin film of water
The pressure on the ice due to skate raises its melting point
Ice is cold

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