Suppose that a client performs an intermixed sequence of push and pop operations on a pushdown stack. The push operations insert the integers 0 through 9 in order onto the stack; the pop operations print the return values. Which of the following sequence(s) could not occur?

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a. 4 3 2 1 0 9 8 7 6 5 b. 4 6 8 7 5 3 2 9 0 1 c. 2 5 6 7 4 8 9 3 10 e. d. 4 3 2 1 0 5 6 7 8 9 1 2 3 4 5 6 9 8 7 0 04 6 5 3 8 1729 1 4 7 9 8 6 5 3 0 2 f. g. h. 2 1 4 3 6 5 87 90