There are two circles that touch each other externally. Radius of the first circle with centre O is 12 cm. Radius of the second circle with centre A is 8 cm. Find the length of their common tangent BC.

A

$$6\sqrt{6}$$ cm

B

$$8\sqrt{3}$$ cm

C

$$8\sqrt{2}$$ cm

D

$$8\sqrt{6}$$ cm