A 4-month-old infant is brought to the pediatrician by his parents because they’re concerned about his frequent respiratory infections, poor feeding habits, frequent vomiting, and colic. The physician notes that the baby has failed to gain expected weight and recommends that the baby have a sweat test performed to detect possible cystic fibrosis. To prepare the parents for the test, the nurse should explain that （）.
A．the baby will need to fast before the test.
B．a sample of blood will be necessary.
C．a low-sodium diet is necessary for 24 hours before the test.
D．a low-intensity, painless electrical current is applied to the skin.