408 A boiler is to be secured in port. After the burners have been secured, the forced draft fan and air registers should be secured __________.

A. immediately after carrying out the former procedures
B. after any oil on the furnace floor has been burned off and cleared of combustion gases
C. after 30 minutes has elapsed, after carrying out these procedures
D. after at least 1 hour has elapsed, after carrying out these securing procedures