Operating System (OS)
To save your computer from viruses you will do

Scan the removable disk before using it
All of these
Install antivirus software and run it
Make physical safe for the removal disk

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Operating System (OS)
A public key encryption system

allows only the correct receiver to decode the data
allows anyone to decode the transmission
None of these
allows only the correct sender to decode the data
does not encode the data before transmitting it

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