a. (4) Compare the CLOCK page replacement algorithm to NRU. What are their strengths and weaknesses? (Assume that LRU is pretty good indicator of likelihood of page use soon.) b. (4) Now devise a way for the CLOCK algorithm to incorporate the optimization used by NRU that CLOCK lacks - how would you have to modify the CLOCK algorithm to add this optimization while still retaining the spirit of CLOCK? Be specific, and justify your changes. c. (2) Why is it that disk block cache systems can implement true LRU, but page replacement systems never do?