2. Suppose that we have a stack S that supports the following operations: PUSH (S, r), which pushes an element z on top of the stack S, Po (S), which removes the element at the top of stack S, and MULTIPoP (S, k), which removes the top k elements from the stack S (if we try to remove more elements than the current number of elements on he stack will be returned). Assume that the stack is initially empty. Give a potential function argument showing that the amortized running time of each of the three stack operations is (1)