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The resistance of a wire is given by R = ρl / πr^2. But m = πr^2 l d, so πr^2 = m / ld.

1. R = ρl^2 / d
2. R = ρl^2 / m
3. R = ρl^2 d / m
4. R = ρl^2 / d^2

Correct answer: R = ρl^2 d / m

Solution

Substituting πr^2 = m / ld into R = ρl / πr^2, we get R = ρl / (m / ld) = ρl^2 d / m.

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