Which factor is most important to ensure that a condensing boiler operates in the condensing mode?

Pick one choice from A to E and explain why.

A$)$ Using only low sulfur fuels

B$)$ Minimizing the entering water temperature to the boiler

C$)$ Maximizing the hot water supply temperature

D$)$ Minimizing the air to fuel ratio

E$)$ Adding nitrous oxide to the fuel mix