2. 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
How many page faults would occur under pure demand paging (all frames are initially empty), assuming four frames are available, under FIFO, LRU, and Optimal Replacement respectively?