The Central Processing Unit $($CPU$)$ is the brain of the computer and uses processor registers to execute a program. Answer the following questions regarding registers.

a$.$ Which processor register stores the address of the next instruction to be executed?

b$.$ Which processor register stores the address of the top byte of the stack?

c$.$ Which processor register stores the address of the byte immediately below the bottom of the stack?

d$.$ Which processor register is used in the fetch portion of the fetch$-$decode$-$execute process of the CPU?

e$.$ Which processor registers are used as a reference location for where variables are stored in main memory?