Determine whether or not the following relations are equivalence relations on the given set. If the relation is an equivalence relation, describe the partition given by it.
((a)) x∼y in R if x≥y;
((b)) m∼n in Z if mn>0;
((c)) x∼y in R if ∣x−y∣≤4;
((d)) m∼n in Z if m≡n(mod6).