Cystic fibrosis is a disease that is passed down through families. It is caused by a defective gene
that makes the body produce abnormally thick and sticky fluid, called mucus. This mucus builds
up in the breathing passages of the lungs and in the pancreas. The buildup of mucus results in
life-threatening lung infections and serious digestion problems. The disease may also affect the
sweat glands and a man's reproductive system. Millions of Americans carry a CF gene, but do
not have symptoms. This is because a person with CF must inherit two defective genes, one from
each parent. About 1 in 29 Caucasian Americans have the CF gene. It is more common among
those of northern or central European descent. Most children with CF are diagnosed by age 2.
For a small number, the disease is not detected until age 18 or older. These children often have a
milder form of the disease.

Which of the following describes the purpose of the passage?
A. To describe the causes of cystic fibrosis
B. To explain how cystic fibrosis is passed down through families
C. To overview the causes of CF and how it is carried
D. To review why some Americans have CF, but others do not