单项选择题

The Internet is an open system, and most information is available to everyone. It is very important to add ______ to existing system for protecting the secret information.

A．web pages
B．modem
C．security
D．network
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单项选择题
As an operating system repeatedly allocates and frees storage space, many physically separated unused areas appear. This phenomenon is called ______.
A．fragmentation
B．compaction
C．swapping
D．paging
单项选择题
In ______, the only element that can be deleted or removed is the one that was inserted most recently.
A．a line
B．a queue
C．an array
D．a stack
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