In a boiler installation the natural draught is produced$?

• A. due to the fact that furnace gases being light go through the chimney giving place to cold air from outside to rush in
• B. due to the fact that pressure at the grate due to cold column is higher than the pressure at the chimney base due to hot column
• C. due to the fact that at the chimney top the pressure is more than its environmental pressure
• D. all of the above