Let \( R \) be a relation on \( \mathbb{N} \times \mathbb{N} \) defined by \( (a,b) R (c,d) \) if and only if \( ad\,(b-c) = bc\,(a-d) \). Then \( R \) is:
Symmetric and transitive but not reflexive
Symmetric but neither reflexive nor transitive
Reflexive and symmetric but not transitive
Transitive but neither reflexive nor symmetric