11. Consider an operating system that uses paging for virtual memory management. Assume that there are 4 page frames which are initially empty. Given the page reference string 1, 2, 3, 4, 5, 1, 3, 1, 6, 3, 2, 3, how many page faults occur if the optimal page replacement is used? (A) 5 (B) 6(C)8(D)9