Lines AB and CD intersect at point O. In clockwise order, name the four angles formed as ∠AOC, ∠COD, ∠DOB, and ∠BOA.
a. (i) Identify the two pairs of vertically opposite angles at O.
b. (ii) State the equality relation that holds for each pair of vertically opposite angles and give a brief reason.
c. (iii) State the relation that holds for the measures of angles that form a linear pair and give a brief reason.
d. (iv) Using your answers to (ii) and (iii), explain why ∠AOC=∠DOB without measuring any angle.