Help would be greatly appreciated. Look at the quadrilateral shown below:

a quadrilateral ABCD is shown with diagonals AC and BD intersecting in point O. Angle AOB is labeled as 1, angle BOC is labeled

Clara writes the following proof for the theorem: If the diagonals of a quadrilateral bisect each other, the quadrilateral is a parallelogram:

Clara'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 to 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 Clara's proof? (6 points)

Select one:
a. CPCTC
b. property of parallelograms
c. transitive property
d. vertical angles theorem