When two circles of radii $$r_{1}$$ and $$r_{2}$$ have their centres at a distance d apart, then length of the common transverse tangent is:

A

$$\sqrt{d^{2} - \left(r_{1} - r_{2}\right)^{2}}$$

B

$$\sqrt{d^{2} + \left(r_{1} - r_{2}\right)^{2}}$$

C

$$\sqrt{d - \left(r_{1} - r_{2}\right)^{2}}$$

D

$$\sqrt{d^{2} - \left(r_{1} + r_{2}\right)^{2}}$$