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（ ）。
{3,5,7,8,1}
{3,4,5,7,8}
{5,7,8}
{3,5,7,9,1}