Suppose that we push integers 0 through 9 in order onto an empty stack. However, a number of topop operations are performed randomly in-between pushes, where topop prints and removes the top item in the stack (top+pop). Which of the following sequences cannot occur? Why? Explain clearly.

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 1 0

d) 4 3 2 1 0 5 6 7 8 9