In an instruction cycle, there are fetch instruction, execute instruction, and check interrupts. Consider some registers in the computer systems. A. PC B. MAR C. MBR D. IR AC F. Interrupt Vector (a) Which registers may be accessed by the memory during instruction fetch? (b) Which registers may be accessed by the CPU during instruction execution? (c) Which registers may be accessed by the CPU when checking interrupts? For the above questions, select the register that may be accessed by listing their labels A-F in alphabetical order with no separators or spaces (e.g., AC). If no registers may be accessed, then answer "none