不是盒图(NS图)的特点。
A．功能域明确
B．灵活转移控制
C．容易确定局部和全程数据的作用域
D．容易表现嵌套关系
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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
A．SELECT*FROM R WHERE E＜'2'
B．SELECT B,E FROM R WHERE E＜'2'
C．SELECT B,E FROM R HAVING E＜'2'
D．SELECT*FROM R WHERE'5'＜B
某系统总线的一个总线周期包含4个时钟周期，每个总线周期中可以传送64位数据。若总线的时钟频率为44MHZ，则总线带宽为(43)。
A．132MB/s
B．88MB/s
C．44MB/s
D．396MB/s
A．ΠA(R)×ΠD(S)
B．R∪S
C．ΠC(R)∩ΠC(S)
D．R｜＞＜｜S
电子商务系统的主体要素是()。
A．Internet
B．企业
C．人
D．商务活动