单选题：A system call is_____ 。

A system call is_____ 。 @[D](1)
A. a user written sub-program
B. a library program in advanced language
C. a command in operating system
D. an interface provided by OS for user program





A.a user written sub-program
B.a library program in advanced language
C.a command in operating system
D.an interface provided by OS for user program


答案：D