Physics
It is more difficult to walk on a sandy road than on a concrete road because

the sand is grainy but concrete is smooth.
the friction between sand and feet is more than that between concrete and feet.
sand is soft and concreter is hard.
the friction between sand and feet is less than that between concrete and feet.

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Physics
Thermohaline circulation is mainly due to

Winds causing circulation of ocean water
Cold, dense ocean water sinking and warmer water replacing it
Warmer, less dense water sinking in the ocean and cold water replacing it
Warm water being added to the ocean due to industial waste

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Physics
A standing wave occurs when

Two waves overlap
The amplitude of a wave exceeds its wavelength
The speed of the wave is zero or near zero
A wave reflects upon itself

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