You are taking care of an 80-year-old woman in the ICU after total right hip replacement. The hospital is located at sea level. Blood gas analysis reveals an arterial oxygen tension of 70 mm Hg and an arterial carbon dioxide tension of 40 mm Hg. She is intubated and breathes 11 breaths a minute of air (FiO2 0.21). What is this patient's A-a gradient?
A. 10 mmHg
B. 20 mmHg
C. 30 mmHg
D. 40 mmHg
E. The A-a gradient cannot be calculated with the information given.