Automation System
An operational amplifier:

can be used to sum two or more signals
None of these
uses the principle of feedback
All of these
can be used to subtract two or more signals

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Automation System
Today, programmable controllers:

can be cost effective even when replacing just a few relays
None of these
are more powerful then the first controllers on the market
All of these
are relatively low cost compared with the original digital equipment available for industrial control

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