Recall the shopping database schema. Consider a list of pairs of product ids (P1, P2), ..., (Pn-1, Pn). Such a list is called pair-distinct if and only if for any element (P1, P2) of the list, it is not the case that (p2, P1) is also an element of the list. Which one of the below Datalog programs expresses the natural language query List all pair-distinct product ids of all products that have been purchased on the same date and for the same unit price"? A(P1, P2) :- purchase(T1, C1, S1, P1, D, Q1, P), purchase(T2, C2, S2, P2, D, Q2, P), P1 P2 ?A(P1, P2) A(P1, P2) :- purchase(T1, C1, S1, P1, D1, Q1, P), purchase(T2, C2, S2, P2, D2, Q2, P), P1 P2 ?A(P1, P2) o A(P1, P2) :- purchase(T1, C1, S1, P1, D, Q1, P), purchase(T2, C2, S2, P2, D, Q2, P) ?A(P1, P2)