A coaxial cable consists of two thin cylindrical conducting shells of radii $$a$$ and $$b$$ $$(a < b)$$. The inductance per unit length of the cable is

A

$$\frac{\mu_0}{2\pi}\frac{(a + b)}{a}$$

B

$$\frac{\mu_0}{4\pi}\ln\left(\frac{a}{b}\right)$$

C

$$\frac{\mu_0}{4\pi}\ln\left(\frac{b}{a}\right)$$

D

$$\frac{\mu_0}{2\pi}\ln\left(\frac{b}{a}\right)$$