Consider the following four schedules due to three transactions (indicated by the subscript) using read and write on a data item x, denoted by r(x) and w(x) respectively. Which one of them is conflict serializable?
(A) r1(x); r2(x); w1(x); r3(x); w2(x)
(B) r2(x); r1(x); w2(x); r3(x); w1(x)
(C) r3(x); r2(x); r1(x); w2(x); w1(x)
(D) r2(x); w2(x); r3(x); r1(x); w1(x)