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

sand is soft and concreter is hard.
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.
the sand is grainy but concrete is smooth.

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Physics
Stationary wave is formed by

Two waves of same frequency travelling in the opposite direction
Two waves of the same speed superposing
Two waves of same frequency travelling in the same direction
A transverse wave superposing a longitudinal wave

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