What occurs if oxygen is turned off last while extinguishing a torch?

Turning off the oxygen last while extinguishing a torch can indeed lead to a dangerous situation. This is primarily because the flame can become unstable if oxygen is cut off while fuel is still flowing. As the torch continues to burn with an improper air-fuel mixture, it can produce soot, an uncontrolled flame, or an explosion risk when the fuel ignites suddenly. The correct sequence of extinguishing the torch is critical for safety.

In contrast, the other options do not reflect the primary concerns related to this practice. While cooling down faster is typically not a concern with the torch's operation, wasting oxygen could be a minor issue but does not encapsulate the potential hazards associated with improper shut-off procedures. Lastly, stating that it has no significant effect downplays the real risks involved, such as blowbacks or flashbacks, which can occur in such situations.