You are about to enter a compartment to investigate a suspected smoldering fire. If you're not wearing an self-contained breathing apparatus, you should test the compartment's atmosphere to determine if there is/are __________.

Correct Answer: Option D — all of the above

Entering a confined space with a suspected fire requires atmospheric testing for oxygen levels, toxic gases, and explosive vapors. Without an SCBA, the atmosphere must be proven safe for human life to prevent asphyxiation or ignition, as smoldering fires often consume oxygen and release lethal carbon monoxide.