Triangle ABC is shown below: *1st image attached*

Given: ΔABC

Prove: All three angles of ΔABC add up to 180°.

The flowchart with missing reason proves the measures of the interior angles of ΔABC total 180°:

*2nd image attached*

Which reason can be used to fill in the numbered blank space?

Alternate Exterior Angles Theorem

Same-Side Interior Angles

Corresponding Angles Postulate

Alternate Interior Angles Theorem