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

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.
the sand is grainy but concrete is smooth.

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Physics
Thermohaline circulation is mainly due to

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
Winds causing circulation of ocean water

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