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

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Physics
Radiocarbon is produced in the atmosphere as a result of

Collision between fast neutrons and nitrogen nuclei present in the atmosphere.
Action of ultraviolet light from the sun on atmospheric oxygen.
Action of solar radiations particularly cosmic rays on carbon dioxide present in the atmosphere.
Lightning discharge in atmosphere.

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