Which statement describes a firewall? A. a program designed to detect and block viruses from infecting your computer
B. a suite of security measures designed to prevent unauthorized access to your computer
C. a software program designed to keep you from accessing important documents on your computer

no backup memory

only #2 represents an object at rest on a table.

in #1, there is no normal force due to the table; the object is accelerating downward.

in #3, the vertical forces are not balanced; the object is accelerating downward.

in #4, neither the vertical nor the horizontal forces are balanced. the object is accelerating upwards and to the right.