一棵二叉搜索树的前序遍历序列为：30, 20, 10, 15, 25, 23, 39, 35, 42，下列哪项对应此二叉搜索树的后序遍历序列？
10, 20, 15, 23, 25, 35, 42, 39, 30
15, 10, 25, 23, 20, 42, 35, 39, 30
15, 20, 10, 23, 25, 42, 35, 39, 30
15, 10, 23, 25, 20, 35, 42, 39, 30
