Which factor is crucial for combustion to sustain after ignition?

The proper fuel/air ratio is crucial for combustion to sustain after ignition because it ensures that there is an optimal mix of fuel and oxygen available to sustain the chemical reaction involved in burning. When this ratio is balanced, the fuel provides enough energy and, in combination with the right amount of oxygen, allows the fire to continue burning effectively.

If the fuel/air ratio is too rich (not enough air), the combustion may become smothered or incomplete, leading to inefficient burning and potential extinguishing of the fire. Conversely, if the ratio is too lean (too much air), there may not be sufficient fuel to sustain the reaction, which can also lead to extinguishment. Therefore, maintaining the proper fuel/air ratio is essential for the continuity of combustion.

The other options mentioned do not contribute directly to sustaining combustion. Carbon dioxide, while a byproduct of combustion, actually acts as a suppressant in higher concentrations. Suppressants are designed to extinguish fires by interrupting the combustion process, and high humidity levels can reduce the availability of oxygen and alter the burning characteristics of fuels, which can hinder the combustion process rather than sustain it.