What should be done first if a flammable gas leak occurs?

In the event of a flammable gas leak, the priority should be to address the situation safely to prevent further hazards. The correct initial action is to close the shutoff valve. This step immediately stops the flow of gas, mitigating the risk of ignition and explosion. Containing the leak is crucial as it helps secure the area and protects lives and property.

Once the shutoff valve is closed, it becomes safer to implement additional measures such as evacuating the area or notifying authorities. While notifying authorities and evacuating are important steps in an emergency response, they should follow the immediate action of stopping the leak to ensure the situation does not worsen. Using a fire blanket is not appropriate in this scenario, as it is designed for extinguishing flames but won't address the gas leak itself.

Therefore, closing the shutoff valve is the most critical first step in managing a flammable gas leak, ensuring safety by controlling the source of the hazard.