1. 9025. Which of the following are fire precautions which must be observed when working on an oxygen system? 1. Display “No Smoking” placards. 2. Provide adequate fire-fighting equipment. 3. Keep all tools and oxygen servicing equipment free from oil or grease. 4. Avoid checking aircraft radio or electrical systems. A. 1, 3, and 4. B. 1, 2, and 4. C. 1, 2, 3, and 4.