Final answer:
The first error in the proof occurs in line 3. The reason provided does not justify the statement because it lacks justification for subtraction.
Step-by-step explanation:
The first error in the proof occurs in line 3.
The reason provided does not justify the statement because it lacks justification for subtraction.
The subtraction property of equality states that if a = b, then a - c = b - c, but the reason in line 3 does not specify what is being subtracted from both sides of the equation.
Therefore, the correct answer is C. It lacks justification for subtraction.