$$\int \frac{dx}{x \sqrt{x^2 - a^2}}$$ is

A

$$\frac{1}{a} \cosec^{-1} \left(\frac{x}{a}\right)$$

B

$$\frac{1}{a} \sin^{-1} \left(\frac{x}{a}\right)$$

C

$$\frac{1}{a} \cos^{-1} \left(\frac{x}{a}\right)$$

D

$$\frac{1}{a} \sec^{-1} \left(\frac{x}{a}\right)$$