Physics
Which phenomenon is responsible for the echo of sound wave?

Refraction
Interference
Reflection
Polarisation

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

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