A composite shaft consisting of two stepped portions having spring constants K₁ and K₂ is held between two rigid supports at the ends. Its equivalent spring constant is

A. K₁ K₂

B. (K₁ + K₂)/ 2

C. (K₁ + K₂)/ K₁ K₂

D. K₁ K₂/ (K₁ + K₂)

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