Consider a quadrilateral ABCD whose four vertices lie on the circumference of a circle.
(i) State the relationship between the measures of the opposite angles of ABCD.
(ii) Explain, with reference to the arcs on which the angles stand, why the relationship stated in part (i) must hold.
(iii) If ∠A=78∘, calculate ∠C and justify your answer using the result from part (i).