A 8yrs old boy with recurrent chest infections and exocrine pancreatic insufficiency is suspected of having cystic fibrosis. Sweat chloride levels have been observed between 40-60 mmol/l on two separate occasions.The next test should be performed to support the diagnosis of Cystic fibrosis is

• A. Demonstrate an abnormal nasal potential difference
• B. Repeat Sweat chloride levels on a different day
• C. Demonstrate an abnormal 72 hour fecal fat collection
• D. Demonstrate an abnormal F508 mutation by DNA analysis