What is the runtime complexity of adding an item to a stack and removing an item from a stack?

1.  Add items to a stack in O(1) time and remove items from a stack on O(n) time.
2.  Add items to a stack in O(1) time and remove items from a stack in O(1) time.
3.  Add items to a stack in O(n) time and remove items from a stack on O(1) time.
4.  Add items to a stack in O(n) time and remove items from a stack on O(n) time.

Correct Answer:

•  Add items to a stack in O(1) time and remove items from a stack in O(1) time.