Theorem: The negative of every irrational number is irrational.
Proof:
1. Suppose there is some irrational number p such that -p is rational.
2. -p = m/n, where m and n are both integers and n ≠ 0.
3. p = -m/n, where -m and n are both integers and n ≠ 0.
4. p is rational, which is a contradiction.
Which of the following is the proper justification for line 2 in the above proof by contradiction?
A. Contradictory supposition
B. Algebra
C. Definition of rational numbers
D. Closure of integers under negation