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A queue is implemented with two stacks: S1 receives every enqueue, and S2 serves every dequeue (the elements are moved from S1 to S2 only when S2 is empty). The operations enqueue(1), enqueue(2), enqueue(3), dequeue, enqueue(4), dequeue are performed in this order. Which element is removed by the second dequeue?
- A2
- B1
- C3
- D4
Correct answer: A