The preorder traversal sequence of a binary search tree is 30, 20, 15, 10, 23, 25, 39, 35, 42. Which one of the following is the postorder traversal sequence of the same tree? Select one O a. 10, 15, 25, 23, 20, 35, 42, 39, 30 O b. 15, 20, 10, 23, 25, 42, 35, 39, 30 O c. 10, 20, 15, 23, 25, 35, 42, 39, 30 O d. 15, 10, 25, 23, 20, 42, 35, 39, 30