Consider a stack S of size n ≥ 1 which is initially empty. In an empty stack first n natural numbers are pushed in sequence and then n pop operations are performed. Push and pop operations take x seconds each. The time elapse between the end of one stack operation and the start of the next operation is y. For m ≥ 1, define the stack-life of m as the time elapsed from the end of push (m) to the start of the pop operation that removes m from stack S. What is the average stack-life of an element of this stack?   

• A. 3y + 2x
• B. n (x + y) - x
• C. n (x + y)
• D. y + 2x