Two closely coiled helical springs with stiffness k₁ and k₂ respectively are connected in series. The stiffness of an equivalent spring is given by

A. (k₁ k₂)/ (k₁ + k₂)

B. (k₁ - k₂)/ (k₁ + k₂)

C. (k₁ + k₂)/ (k₁ k₂)

D. (k₁ - k₂)/ (k₁ k₂)

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