Let the address stored in the program counter be designated by the symbol X$1.$ The instruction stored in X$1$ has an address part $($operand reference$)$ X$2.$ The operand needed to execute the instruction is stored in the memory word with address X$3.$ An index register contains the value X$4.$ What is the relationship between these various quantities if the addressing mode of the instruction is $($a$)$ direct; $($b$)$ indirect; $($c$)$ PC relative; $($d$)$ indexed?