Which one of the following is not an equivalence relation?
1
R1 on Z defined by a R1 b ⇔ a - b is divisible by m, where m is a fixed positive integer.
2
R2 on R defined by a R2 b ⇔ 1 + ab > 0 for all a, b ∈ R.
3
R3 on N x N defined by (a, b) R3 (c, d) ⇔ ad = bc for all a, b, c, d ∈ N.
4
R4 on Z defined by aR4 ⇔ a - b is an even integer for all a, b ∈ z.
5
Question Not Attempted