The pre-order traversal of a binary search tree is given by $12, 8, 6, 2, 7, 9, 10, 16, 15, 19, 17, 20$. Then the post-order traversal of this tree is
- $2, 6, 7, 8, 9, 10, 12, 15, 16, 17, 19, 20$
- $2, 7, 6, 10, 9, 8, 15, 17, 20, 19, 16, 12$
- $7, 2, 6, 8, 9, 10, 20, 17, 19, 15, 16, 12$
- $7, 6, 2, 10, 9, 8, 15, 16, 17, 20, 19, 12$