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.
1
\( {6 \ \sqrt{6} }\) cm
2
\( {8 \ \sqrt{3} }\) cm
3
\( {8 \ \sqrt{2} }\) cm
4
\( {8 \ \sqrt{6} }\) cm