The skin depth $\delta$ in a good conductor of conductivity $\sigma$ and permeability $\mu$ at frequency $\omega$ is:
(A)
\sqrt{\frac{2}{\omega \mu \sigma}}
(B)
\sqrt{\frac{\omega \mu}{\sigma}}
(C)
\frac{1}{\omega \mu \sigma}
(D)
\sqrt{\frac{2 \sigma}{\omega \mu}}