What Test Is Most Useful for the Diagnosis of Asthma $\dot { V } / \dot { Q }$

What test is most useful for the diagnosis of asthma in the symptom-free patient?

A) arterial blood gases
B) bronchoprovocation testing with pulmonary function tests
C) chest radiograph
D) ventilation-perfusion ratio ( $\dot { V } / \dot { Q }$ ) scans