A disk scheduling mechanism in an operatingsystem causes the disk arm to sweepback and forth across the disk surface servicing all requests in its path. This is a

A．First Come First Serve

B．Shortest Seek Time First

C．Scan

D．None of the above

化工生产要求实现连续不间断生产，但实际运行中短时的局部子系统或操作单元停车或者总系统（）不可避免。

A.临时停车

B.计划停车

C.临时开车

D.计划开车