Question

The figure below shows a rectangle ABCD having diagonals AC and DB.

A rectangle ABCD is shown with diagonals AC and BD.
Zinnia wrote the following proof to show that the diagonals of rectangle ABCD are congruent.

Zinnia's proof:
Statement 1: In triangle ADC and BCD, AD = BC (opposite sides of a rectangle are congruent)
Statement 2: Angle ADC = Angle BCD (angles of a rectangle are 90°)
Statement 3: DC = DC (reflexive property of equality)
Statement 4: Triangle ADC and BCD are congruent (by ASA postulate)
Statement 5: AC = BD (by CPCTC)

Which statement in Zinnia's proof has an err