When shutting down a boiler for internal cleaning, you should:
a. Shut down the boiler, then fill it completely with chemically treated water
b. Shut off the fire, drain, open up and flush
c. Allow the boiler to cool; then drain, open up and flush it
d. Shut off the fire and flush with cold water to cool it down
e. Let it stand for a couple of days then drain
Answer: c