Look at the quadrilateral shown below

Clara writes the following proof for the theorem if the diagonals of a quadrilateral bisect each other the quadrilateral is a paralelogram

Ciara's proof

1. For triangles AOB and COD, angle 1 is equal to angle 2 as they are vertical angles
2. AO = OC and BO = OD because it is given that diagonals bisect each other
3. The triangles AOB and COD are congruent by SAS postulate
4. Similarly, triangles AOD and COB are congruent 5. By angle ABD is equal angle CDB and angle ADB is equal to angle CBD
6. As the alternate interior angles are congruent, the opposite sides of quadrilateral ABCD are parallel
7. Therefore ABCD is a parallelogram

Which is the missing phrase in Ciara's proof?

A) CPCTC
B) property of parallelograms
C) transitive property
D) vertical angles theorem