🔍 Key Concepts

• Cartridge‑operated dry chemical extinguisher construction • Difference between an expellant gas and the actual extinguishing agent • Common gases used in small pressure cartridges on portable extinguishers

💭 Think About

• Look at the illustration: is the main cylinder holding the extinguishing agent or the propellant gas? Where would the small cartridge (#4) fit into that system? • Think about which of the listed gases is most commonly stored in small metal cartridges used to pressurize portable extinguishers, especially older style cartridge‑operated units. • Eliminate any choices that are typically used as the main extinguishing agent rather than just as the propellant gas.

✅ Before You Answer

• Verify which of the options is normally used as an expellant gas in a sealed cartridge on dry chemical extinguishers. • Ask yourself which gases in the list are usually stored in small steel cartridges rather than large cylinders with discharge horns. • Be sure you are not confusing the gas used to pressurize the extinguisher with the substance that actually puts out the fire.