a b c d

b

$ \frac{R}{l} = \frac{\rho}{ A }=\frac{4\times 10^{-7}}{ 8\times 10^{-6}}= 0.05 \Omega/m$ $ \text{Potential Gradient }\frac{V}{l} = \frac{IR}{l} = 25mV/m$