This scenario suggests a potential multiple-casualty incident with a possible environmental hazard, such as carbon monoxide or another toxic gas. NREMT places strong emphasis on scene size-up as the first and most critical step in EMS operations.

Option B is correct because determining the size and scope of the incident allows the EMT to assess scene safety, identify hazards, estimate the number of patients, and determine the need for additional resources. Entering the scene without this assessment could result in responder injury or death.

Option A is incorrect because establishing command typically follows an initial size-up. Command cannot be effectively established without understanding the nature of the incident.

Option C is incorrect because announcing the presence of poisonous gas without confirmation may cause panic and is not an appropriate first action.

Option D is incorrect because attempting rescue before confirming scene safety violates NREMT’s core principle that rescuer safety comes first.

NREMT standards clearly state that EMTs must never enter a potentially hazardous scene until it has been properly assessed and deemed safe or appropriate resources are requested.