Inside Job Deduction Assignment Answers

Question:

Assuming that it wasn't an inside job, then the lock was forced unless the thief stole the key. The thief didn't steal the key. We may infer that the robbery was an inside job, inasmuch as the lock wasn't forced. --What is the conclusion of this argument?

Inside Job Deduction Answer and Explanation

3. Based on 1 and 2, the only remaining option is that the lock was forced.

4. However, the argument states that the lock wasn't forced.