The pre order traversal of binary tree is 40, 20, 10, 30, 60, 50, 70. Which one of the following is the post-order traversal of the tree?

The pre order traversal of binary tree is 40, 20, 10, 30, 60, 50, 70. Which one of the following is the post-order traversal of the tree?

1. a) 10,20,30,40,50,60,70
2. b) 10,30,20,50,70,60,40
3. c) 40,20,60,10,30,50,70
4. d) 70,50,60,30,10,20,40