The length of the direct common tangent to two circles of radii $$r_1$$ and $$r_2$$ and is the distance between their centres is:

A

$$\sqrt{d^2(r_1r_2)}$$

B

$$\sqrt{d^2 - (r_1^2r_2^2)}$$

C

$$\sqrt{d^2 - (r_1 + r_2)^2}$$

D

$$\sqrt{d^2 - (r_1 - r_2)^2}$$