Let \( R \) be a relation on \( \mathbb{R} \), given by \( R=\left\{(a,b):3a-3b+\sqrt{7}\ \text{ is an irrational number}\right\} \).Then \( R \) is :
reflexive and transitive but not symmetric
reflexive and symmetric but not transitive
an equivalence relation
reflexive but neither symmetric nor transitive