This instruction works as follows: R1[mem11-[mem2] In a multi-cycle datapath implementation, this instruction will: a. Use the MDR twice b. Use the ALU once c. Use the "shift to left" unit twice d. None of the above 18. Consider the following hypothetical instruction: MemS mem1, R1, mem2 This instruction works as follows: [mem1]R1-[mem2] One of the following is correct about this instruction: 19. By comparing the hypothetical instructions given in Questions (17) and (18), if we run these instructions on the same processor, then one of the following is correct: a. Both instructions have the same CPI b. MemS executes faster than SubMem c. SubMem executes faster than MemS d. Mems does not need the MDR 20. The CPI for the instruction SubMem in Question (17) is: a. 10 b. 11 c. 12 d. 13