单选题：Given a page reference sequence as: 1，3，7，3，4，2， 3，2，6，2，1，5，6，2

Given a page reference sequence as: 1，3，7，3，4，2， 3，2，6，2，1，5，6，2，1. Suppose the system has only 4 frames and all 4 frames are initially empty. Answer the following three questions.
Using the optimal replacement algorithm, how many page faults?@[B](1)
A. 6
B. 7
C. 8
D. 9




A.6
B.7
C.8
D.9


答案：B