Amrita, 27 yrs, presents with a history of progressive breathlessness and exercise intolerance since four months.Her FVC is 90% and FEV1/FVC is 86%.Oxygen saturation after exercise was observed to drop from 92% to 86%.What is the likely diagnosis:

• A. Primary pulmonary hypertension
• B. Primary alveolar hypoventilation
• C. Interstitial lung disease
• D. Anxiety disorder