【单选题】

User programs interact with the kernel through a set of standard (71) . They request services to be provided by the kernel. Such services would include accessing a file: open close, read, write, link, or execute a file; starting or (72) accounting records; changing ownership of a file or (73) ; changing to a new directory; creating, (74) , or killing a process; enabling access to hardware devices; and setting limits on system resources. Unix is a multi -user, multi -tasking operating system. You can have many users logged into a system simultaneously, each running many programs. It’s the kernel’s job to keep each process and user separate and to regulate access to (75) , including cpu, memory, disk and other L/O devices.

A．system commands
B．system transfer
C．system calls
D．system rings