In the Section "Investigating Life: Does Natural Selection Maintain Cystic

In the section "Investigating Life: Does Natural Selection Maintain Cystic Fibrosis?",what is the independent variable in the figure?

A) the number of defective copies of the CFTR gene in humans
B) the number of defective copies of the CFTR gene in mice
C) the amount of fluid in the intestines
D) whether or not the subjects were exposed to cholera toxin
E) whether or not the subjects were exposed to Vibrio cholera bacteria