13. Which of the following system calls is NOT for file system? A. read B. write C. exit D. open Answer: 14. Which of the following is a deadlock? A. a running program in endless loop B. Several programs are in a condition of circularly waiting for resources C. a process in a ready queue D. a process is blocked since waiting for a message Answer: 15. Which of the following is the meaning of the term "busy waiting"? A. A process is waiting for the CPU resource B. A process is waiting for the devices resource C. A process is waiting for the I/O bus resource D. A process is waiting for a condition to be satisfied in a tight loop without relinquishing the processor Answer: 16. Which of the following page replacement algorithms is not a practical algorithm, only for algorithm comparison? A. LRU algorithm B. LRU-Approximation C. Optimal algorithm D. LFU