A 32-year-old woman with type 1 diabetes mellitus presents to the emergency department with 2 days of nausea, vomiting, and diffuse abdominal pain. On exam she is tachypneic with deep, rapid respirations. Laboratory studies reveal: serum glucose 520 mg/dL, Na+ 138 mEq/L, K+ 4.5 mEq/L, Cl– 98 mEq/L, HCO3– 12 mEq/L. Arterial blood gas shows: pH 7.25, PaO2 95 mmHg. Which of the following PaCO2 values best represents appropriate respiratory compensation for her primary acid-base disorder?