A queue is implemented using two stacks. Consider the following scenarios for…

1. A.

   If enqueue (push) is O(1), then dequeue (pop) is O(n); if enqueue (push) is O(n), then dequeue (pop) is O(1)

2. B.

   If enqueue (push) is O(1), then dequeue (pop) is O(1); if enqueue (push) is O(n), then dequeue (pop) is O(n)

3. C.

   Both enqueue and dequeue are always O(n)

4. D.

   Both enqueue and dequeue are always O(1)

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