A conductor of diameter d and length l has a resistance of R Ω. If the diameter is halved and the length doubled, what will its resistance become?

Correct answer D. 8R Ω

Explanation

Resistance R = ρl/A. Halving the diameter cuts the cross-sectional area to one quarter, multiplying R by 4, and doubling the length multiplies it by 2 again, giving 8R.