Let the address stored in the program counter of a computer be designated by the symbol X1. The instruction stored in location X1 had an address part X2. The operand needed to execute the instruction is stored in the memory word whose address is X3. An index register contains the value X4. What should be the relationship between the various addresses if the addressing mode of the instruction is (a) direct; (b) indirect; (c) relative; (d) indexed