已知两个序列 s1={1,3,4,5,6,7,7,8,1}、s2={3,5,7,4,8,2,9,5,1}，则它们的最长公共子序列是（ ）。 A. {3,5,7,8,1} B. {3,4,5,7,8} C. {5,7,8} D. {3,5,7,9,1}