Oxygen lines must be free of grease or oil due to the fire hazards they pose. Any contamination can significantly increase the risk of combustion when oxygen is under pressure. Therefore, the correct answer is B. Grease or oil.
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Oxygen lines should be free of grease or oil to prevent fire hazards. Contaminants can cause dangerous reactions under pressure. While stainless steel and rubber are typical materials used, the main concern is cleaning from oils and greases. ;