a) It is a computer system in which exactly the same processing is performed in two
computers, and its reliability is assured by cross-checking the results.
b) It is a computer system that is composed of a primary system used usually and a
secondary system kept on standby in preparation for a failure.
c) It is a distributed processing system in which the computers in a group connected via
a network have an equivalent relationship.
d) It is a distributed processing system that provides a clear hierarchical relation to the
computers in a group connected via a network.