Use four page frames and the following reference string: 1, 2, 3, 4, 5, 3, 4, 1, 6, 7, 8, 7, 8, 9, 7, 8, 9, 5, 4, 5, 4, 2. show how many page faults occur using the following page replacement algorithms: a) fifo b) lru c) optimal