单选题：When does preemption take place?

When does preemption take place? @[A](1)
A. When a quantum (time slice) expires.
B. When a process issues an I/O request.
C. When a process exits.
D. All of the above.





A.When a quantum (time slice) expires.
B.When a process issues an I/O request.
C.When a process exits.
D.All of the above.


答案：A