Operating systems use stacks for processing of subroutine calls and their returns. Suppose we have a Main program and three subroutines a, b, and c, in such a way that Main calls a, a calls b, and b...

Operating systems use stacks for processing of subroutine calls and their returns. Suppose we have a Main program and three subroutines a, b, and c, in such a way that Main calls a, a calls b, and b calls c. The calling procedure passes the return address to the called procedure. These addresses are stored in a stack. If the return address to Main, a, b, and c are q, r, s, and t respectively, show the state of the stack when:

a. c is executing

b. after b has run