A circular clock face has a radius of cm. A smaller circular disk with a radius of cm is externally tangent to the clock face at o'clock. The disk has an arrow painted on it, initially pointing in the downward vertical direction. If the disk rolls clockwise around the clock face, at what point on the clock face will the disk be tangent when the arrow is next pointing in the upward vertical direction?
6 o'clock
7 o'clock
8 o'clock
9 o'clock
10 o'clock
