2. Suppose that a client performs an intermixed sequence of (stack) push and pop operations. The push operations push the integers 0 through 9 in order on to the stack; the pop operations print out the return value. Circle the following sequence(s) that could not occur. (20 Points) a. 4321098765 b. 2143658790 c. 0465381729 d. 468753290 1