Computer Networking
X.21 is

a very common bit-oriented data link protocol issued by ISO
a method access control technique for multiple-access transmission media
None of these
a method of determining which device has access to the transmission medium at any time
network access standard for connecting stations to a circuit-switched network

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Computer Networking
An encyclopedic database

None of these
All of these
is generally free
is an information utility that specializes in storing and searching information
is easy for beginners to use

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