In the IBM/370, memory protection is provided through the use of keys. A key is a 4-bit quantity. Each 2-K block of memory has a key (the storage key) associated with it. The CPU also has a key (the protection key) associated with it. A store operation is allowed only if both keys are equal or if either is 0. Which of the following memory-management schemes could be used successfully with this hardware?
$a$. Bare machine
$b$. Single-user system
$c$. Multiprogramming with a fixed number of processes
$d$. Multiprogramming with a variable number of processes
$e$. Paging
$f$.Segmentation
