Two circles having radii 12 cm and 8 cm, respectively, touch each other externally. A common tangent is drawn to these circles which touch the circles at M and N, respectively. What is the length (in cm) of MN?

A

$$8\sqrt{8}$$

B

$$8\sqrt{6}$$

C

$$6\sqrt{8}$$

D

$$6\sqrt{6}$$