A process contains six virtual pages on disk and is assigned a fixed allocation of four (4) frames in the main memory. Consider the following virtual page reference string: 1,2,3,0,4,0,4,3,1,2,0,5,2,4,1 a. How many page faults would occur if Optimal policy is used? Show clearly when each page fault would occur. (4 marks) b. How many page faults would occur if FIFO policy is used? Show clearly when each page fault would occur. (4 marks) c. How many page faults would occur if LRU policy is used? Show clearly when each page fault would occur. (4 marks) d. Explain the limitation of Optimal policy. (2 marks)