Let AB be a diameter of a circle with centre O and length 10 cm. Tangents are drawn to the circle at A and B.
(i) Prove that the two tangents are parallel.
(ii) Find the distance between these two tangents.
(iii) A point C is taken on the tangent at A such that AC=6 cm. Find the length of the perpendicular drawn from O to the line AC and justify your result.