Arielle is an 8-year-old with cystic fibrosis. For 30 minutes each morning and evening, a parent holds her body in various positions and massages and kneads the areas over her lungs to dislodge the abundant, sticky mucus, which Arielle spits into a jar. She also takes medication - an enzyme - that thins the mucus. If she didn't take these measures, the mucus in her lungs would most likely increase her risk of
A. cancer.
B. emphysema.
C. infection.
D. asthma.
E. mutation.