Please help me with these questions!

When prove that all positive integers less than 4 are prime numbers by showing that 1, 2 and 3 are prime numbers, which of the following kinds of proof are we using? Proof by contradiction Proof by induction Trivial proof Exhaustive proof Theorem: Given p - q, or - 7q, -r, prove -p A 7q Proof: 1. p - q Premise 2. 75 - 79 Premise 3. q -r From Line 2 4. p - r From Lines 1 and 3 5. or Premise 6. -p From Lines 4 and 5 7. -q From Lines 2 and 5 8. -p A -q From Lines 6 and 7 In the proof shown above, what is the correct justification in the fourth step? Modus tollens Contraposition O Hypothetical syllogism Modus ponens