10. 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?

S_{1} : r_{1}(X); r_{2}(X); w_{1}(X); r_{3}(X); w_{2}(X)

S_{2} : r_{2}(X); r_{1}(X); w_{2}(X); r_{3}(X); w_{1}(X)

S_{3} : r_{3}(X); r_{2}(X); r_{1}(X); w_{2}(X); w_{1}(X)

S_{4} : r_{2}(X); w_{2}(X); r_{3}(X); r_{1}(X); w_{1}(X)