The stiffness of a closely coiled helical spring subjected to an axial load W is equal to (where G = Modulus of rigidity, and n = Number of active coils)

A. Gd⁴/ 8D3n

B. Gd3/ 8D3n

C. Gd⁴/ 4D3n

D. Gd⁴/ 8D²n

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