1. PLCs were originally designed as replacements for: a) microcomputers. c) analog controllers. b) relay control panels. d) digital controllers.

PLCs (Programmable logic controller) were original designed as replacement for relay control panels.  Relay is another word for a switch. Relay control panels were made for the control of electronic instrumentents or devices. Later on PLCs were made to replace these timers and relay control panels. PLCs are more reliable, flexible and can work better in the harsh environmental situations then the relay control panels.