In a child with respiratory distress,failure to thrive.His sweat chloride leveles were estimated 35 meq/L and 41 meq/L.what is the next best test to do cystic fibrosis for diagnosis-

• A. DNA analysis (delta)F 508 mutation
• B. Nasal transmembrane potential difference
• C. 72 hour fecal fat estimation
• D. CT chest