The missing reason in step 2, where you are stating that $CD \cong CD$, is known as the Reflexive Property of Congruence. This property states that any geometric figure (in this case, segment $CD$) is congruent to itself.

Updated reasoning for step 2:

• Statement: $CD \cong CD$
• Reason: Reflexive Property of Congruence

Would you like me to explain further steps in the proof or dive deeper into this problem?

Here are 5 related questions:

1. What is the significance of the reflexive property in geometric proofs?
2. How does the SAS (Side-Angle-Side) congruence criterion apply to this problem?
3. What other properties of congruence are commonly used in geometric proofs?
4. Why is it important to establish that $AB \parallel EF$?
5. Can triangle congruence alone prove lines are parallel, or do we need additional theorems?

Tip: Always check for shared sides or angles between triangles, as reflexive properties or vertical angles can be crucial in many geometric proofs.