19. 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 for the following replacement algorithms? Assuming physical memory of four frames? Remember all frames are initially empty, so your first unique pages will all cost one fault each. a) LRU replacement b) FIFO replacement c) Optimal replacement