When you move the mouse pointer to a（），the pointer’s shape usually changes to a pointing hand.

单项选择题（）is a copy of a file for use in the event of failure or loss of the original.

A.second storage
B.buffer
C.backup
D.database

单项选择题（）is the basic software that allows the user to interface with the computer.

A.Display
B.Application
C.Screen
D.Operating System