Which of the following is suitable as a description of a duplex system?
a) A duplex system consists of two computer systems. Normally, one of these systems is used as an online system, while the other is used as a standby system for batch processing, development work, and the like.
b) A duplex system has higher reliability than a dual system, but is expensive.
c) A duplex system has redundant hardware, and the same data is processed in parallel by two processing systems, the results of which are collated at fixed time intervals.
d) In a duplex system, two CPUs share main memory and external storage devices, and operate on a single OS. Processes are handled in parallel by each CPU, and the load is controlled so that it can be distributed.