Consider the following page reference string: 1, 2, 3, 4, 2, 1, 5, 6, 2, 1, 2, 3, 7, 6, 3, 2, 1, 2, 3, 6 Specify throughout the execution sequence which pages are present in a part of the physical memory and the number of page faults using the following algorithms: a) FIFO replacement b) OPTIMAL replacement c) LRU replacement d) MRU replacement