You are answering a USMLE-style medical question. Question: A 51-year-old woman goes to respiratory function testing in order to evaluate her shortness of breath. She recently joined a walking program with her friends in order to lose weight; however, she noticed that she was not able to keep up with her friends during the program. She has a 60-pack-year history of smoking as well as hypertension well-controlled on medication. The following represent the parameters for this patient's respiratory anatomy and function: Vital capacity (VC): 5,000 mL Inspiratory reserve volume (IRV): 2,500 mL Functional reserve capacity (FRC): 2,700 mL Residual volume (RV): 1,000 mL Upper airway volume: 100 mL Conducting airway volume: 125 mL Inspired CO2 pressure (PiCO2): 1 mmHg Arterial CO2 pressure (PaCO2): 50 mmHg Expired CO2 pressure (PeCO2): 20 mmHg Which of the following best represents the total volume of gas in this patient's airways and lungs that does not participate in gas exchange? Options: A) 480 mL B) 600 mL C) 800 mL D) 1200 mL Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.