**Core Concept**
The pin index system is a safety mechanism used to prevent incorrect connections of medical gas cylinders. It involves a specific arrangement of pins on the yoke of the medical gas outlet that correspond to specific holes on the gas cylinder, ensuring that the correct gas cylinder is used for a particular medical gas outlet. In the context of the question, the pin index of O2-Co2 (CO2 < 7.5%) refers to the specific arrangement of pins on the yoke of the medical gas outlet that corresponds to a CO2 cylinder with a concentration of less than 7.5%. **Why the Correct Answer is Right** The correct pin index for O2-Co2 (CO2 < 7.5%) is based on the standard pin index system used in medical gas cylinders. CO2 cylinders with a concentration of less than 7.5% have a specific pin arrangement that corresponds to the yoke of the medical gas outlet. This ensures that the correct cylinder is used for the medical gas outlet, preventing potential harm to patients and medical staff. The specific pin arrangement for CO2 cylinders is designed to prevent incorrect connections with other medical gas cylinders, such as oxygen or nitrous oxide cylinders. **Why Each Wrong Option is Incorrect** **Option A:** This option is incorrect because it does not specify the correct pin index for O2-Co2 (CO2 < 7.5%) medical gas cylinders. **Option B:** This option is incorrect because it is not the correct pin index for O2-Co2 (CO2 < 7.5%) medical gas cylinders, and may correspond to a different type of medical gas cylinder. **Option C:** This option is incorrect because it does not match the correct pin index for O2-Co2 (CO2 < 7.5%) medical gas cylinders. **Clinical Pearl / High-Yield Fact** When using medical gas cylinders, it is essential to ensure that the correct cylinder is used for the specific medical gas outlet to prevent potential harm to patients and medical staff. The pin index system is a critical safety mechanism that helps to prevent incorrect connections. Always verify the pin index of the medical gas outlet and the cylinder before connecting them. **Correct Answer:** C. 4