Automation System
There are several procedures you can implement to provide maximum security. The most important of these is:

Verify the reputation of the people installing your system
run regular simulated tests using TARP (testing of Applications Recovery Planes)
None of these
do not leave training manuals unsecured
do not allow unbonded service people to work on your equipment

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Automation System
D/A (digital to analog) conversion:

None of these
has to be sampled at 1/2 the highest input frequency
must be fed through a filter to reproduce the original signal accurately
both (b) and (c)
results in a binary code as the final output

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