Stack effect is the movement of air in and out of buildings, chimneys, flue gas stacks, or other containers, resulting from air buoyancy. Stack losses typically are the largest single energy loss in boiler operations and involve both excess air and stack temperatures. The amount of heat lost will depend on the temperature and volume of gas leaving the boiler. Some stack-gas heat losses are unavoidable but controlling the excess air or lowering the gas temperature as close to the air temperature surrounding the boiler can improve overall efficiency. So, the correct answer is option A.