The maximum efficiency of spiral gears is (where θ = Shaft angle, and φ = Friction angle)

A. sin (θ + φ) + 1/ cos (θ - φ) + 1

B. cos (θ - φ) + 1/ sin (θ + φ) + 1

C. cos (θ + φ) + 1/ cos (θ - φ) + 1

D. cos (θ - φ) + 1/ cos (θ + φ) + 1

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