Geometry

Question Analysis:
This question tests how to begin a proof by contradiction.
The focus is on assuming the negation of the conclusion.

Key Concept Explanation:
In contradiction-based indirect proofs, we assume the conclusion is false, and then show that this leads to an impossible or illogical outcome.

Step-by-Step Solution:
The original statement:
If angle A and B are supplementary and congruent, then each must be 90°.
To use contradiction, we assume the conclusion is false:
→ The angles are not 90°.
Then we show this contradicts the given:
If the angles are congruent and supplementary, then m∠A + m∠B = 180°, and m∠A = m∠B,
so each angle must be 90°.
Thus, assuming otherwise leads to a contradiction.

Option Analysis:
A. Incorrect — assumes part of the conclusion but not fully.