数据库系统的三级模式结构是指（）。

Messages that are sent electronically over a network．

(72)A series of instructions(commands) performed in a sequence specifying actions to accomplish a task．

(73)The character code built into most modern personal computers．

(74)The minimal unit of information，it can have the value 1 or 0．

(75)The brains of a computer, it contains the ALU，the clock，many registers，the bus interface，and circuitry that makes these parts work together．

A．input

B．output

C．data

D．E-Mail

Which of the following is true？

A．No firewall contains serious exploitable bugs.

B．Your computer can't work without a firewall.

C．Direct Internet connections provide hackers with an easy way of invasion.

D．Firewalls need no configuration.

A computer tape which contains historical project information is called ____ tape.

A.Database

B.Baseline.

C.Archive.

D.Lessons learned.

E.Life cycle summary.

From the last paragraph we know that ______.

A．once a computer has been programmed, it can do as much as one wishes

B．some instructions are embedded in some specialized automobile engines

C．programs can be built into hardware itself but can't exist independently

D．a general purpose computer contains some built in programs; therefore, it doesn't need external programs

A complete computer system contains(74)。

A．arithmetic unit, control unit and memory

B．input/output devices and mainframe

C．mainframe. and programs of utility

D．hardware system and software system

Which of the following about a computer virus is not true？

A．It is a piece of software whose purpose is to replicate.

B．It is a program that gains control over its host in various ways.

C．It all contains code which won't be activated without a trigger mechanism.

D．It sometimes attaches to a file.

A．computers are likely to become a new form. of intelligent life

B．human beings have lost control of computers

C．the intelligence of computers will eventually surpass that of human beings

D．the evolution of intelligence will probably depend on that of electronic brains

A．computers are likely to become a new form. of intelligent life

B．human beings }lave lost control of computers

C．the intelligence of computers will eventually surpass that of human beings

D．the evolution of intelligence will probably depend on that of electronic brains