Let \( A=[2,3,6,8,9,11] \) and \( B=[1,4,5,10,15] \). Let \( R \) be a relation on \( A\times B \) defined by \( (a,b)R(c,d) \) if and only if \( 3ad-7bc \) is an even integer. Then the relation \( R \) is:
reflexive and symmetric but not transitive
an equivalence relation
reflexive but not symmetric
transitive but not symmetric