Before inserting the breathing hose in your mouth, what must you do?

The correct action before inserting the breathing hose in your mouth is to pull the plug out. This step is crucial because it ensures that the breathing apparatus is fully operational and that air can flow through the system. The plug typically seals the airway, and removing it allows for proper inhalation of air that is filtered and safe to breathe in. This action is foundational for ensuring effective usage of the equipment during emergency situations in coal mining or other hazardous environments.

Other choices, while they relate to the overall operation of the system, do not directly address the immediate prerequisite for using the breathing hose. Closing the valve might prevent airflow, turning on the SCSR is necessary for general functionality, and checking the indicator could be part of pre-use checks but does not specifically prepare the device for immediate use. Therefore, pulling the plug is a critical step in preparing the breathing apparatus for operation.