A temperature control system consist of four thermostats controlling three heating units. The thermostat contacts are set to close at 50, 60, 70 and 80 degrees Fahrenheit, respectively. The PLC ladder logic program is to designed so that a temperature below 50 degree F, three heaters are be to be on. Between 50 60 60 degree F two heater are to be on. For 60 to 70 degree F, one heater is to be on. Above 80 degree F, there is a safety shutoff for all three heaters in case one stays on because of malfunction. A master switch is to be used to turn the system on and off. Prepare a typical PLC program for this control process.