Which two signs may indicate possible structural collapse in a burning building?

Severe sagging of floors and audible cracking sounds are critical indicators of potential structural collapse within a burning building. When floors sag, it suggests that the structural integrity is compromised, possibly due to fire weakening the support beams and materials. This is a clear sign that the weight distribution has been altered, which can lead to a collapse.

Audible cracking sounds are equally important; these noises often indicate that the materials are under stress and may be close to failure. This can happen when heat affects the structural elements, and the integrity of walls, floors, and ceilings is at risk. Both signs are essential for firefighters to recognize, as they provide a warning that immediate action may be necessary to ensure the safety of personnel within or adjacent to the structure.

The other choices, while they may present situational awareness in a fire scenario, do not directly indicate structural instability like sagging and cracking do. For instance, smoke rising and light flickering might reflect fire intensity rather than structural health, while temperature drops and wind changes vary independently of a building's structural integrity. Quietness and the absence of flames may suggest that a fire is either extinguished or not well-ventilated, but do not serve as reliable indicators of potential collapse.