Determine which of these relations are symmetric. the variables x, y represent integers. da. (x, y) e r if and only if x â y is positive. ob. (x, y) = r if and only if xy is negative. oc. (x, y) e r if and only if x + y is even. od. (x, y) r if and only if x = lyl. de. (x, y) e r if and only if x +y is positive.