Physics
A system exhibiting S.H.M. must possess

Elasticity only
Inertia only
Elasticity as well as inertia
Elasticity, inertia and an external force

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

the friction between sand and feet is less than that between concrete and feet.
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.

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