Operating System - Old Questions

What is belady's Anomaly in page replacement Algorithm? Explain with example. Consider the following page reference string: 7,2,3,1,2,5,3,4,6,7,7,1,0,5,4,6,2,3,0,1. How many page faults will occur if the program has four page frames available to it and uses the following page replacement algorithm? a)FIFO replacement   b) LRU replacement    c) Optimal  replacement