Final answer:
The first step in beginning triage at the scene of a mass-casualty incident, such as a train derailment, is to identify all Priority 1 patients. Priority 1 patients are those who have life-threatening injuries and require immediate attention.
Step-by-step explanation:
The first step in beginning triage at the scene of a mass-casualty incident, such as a train derailment, is to B. Identify all Priority 1 patients. Priority 1 patients are those who have life-threatening injuries and require immediate attention. By identifying these patients first, you can ensure that they receive the necessary medical care as quickly as possible.