A. (1/2). μ W (r₁ + r₂)
B. (2/3). μ W (r₁ + r₂)
C. (1/2). μ W [(r₁³ - r₂³)/(r₁² - r₂²)]
D. (2/3). μ W [(r₁³ - r₂³)/(r₁² - r₂²)]
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