Automation System
Relay ladder logic is used in a PC because:

most industrial control personnel are familiar with relay ladder logic diagrams and this is the easiest for them to use
it is the best industrial control language ever developed.
None of these
it is impossible to write a control program for a machine in any other logic
All of these

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

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

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