A coil is made from $N$ turns of insulated copper wire wrapped around a cylindrical iron core whose diameter is $D$. The copper wire has diameter $d$. The resistivity of copper is $\rho$. Diameter $D$ is much greater than diameter $d$. What is the total resistance between the two ends of the coil?
- A$\frac{4N\rho D}{d^2}$
- B$\frac{4N\rho d}{D^2}$
- C$\frac{8N\rho D}{d^2}$
- D$\frac{8N\rho d}{D^2}$