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 ( ? , ? ) ? ( ? , ? )    ⟺    3 ? ? − 7 ? ?  is an even integer. (a,b)R(c,d)⟺3ad−7bc is an even integer. Then the relation $R$ is: