A two$-$word instruction is stored in memory at an address designated by the symbol W$.$ The address field of the instruction $($stored at W $1)$ is designated by the symbol Y$.$ The operand used during the execution of the instruction is stored at the effective address symbolized by Z$.$ An index register contains the value X$.$ State how Z is calculated from the other addresses if the addressing mode of the instruction is $($a$)$ direct; $($b$)$ indirect; $($c$)$ relative; $($d$)$ indexed.