Figuer: -When Ellison and Gotelli Manipulated Concentrations of Nutrients Available to Nutrients

Figuer:
-When Ellison and Gotelli manipulated concentrations of nutrients available to pitcher plants,what was the independent variable?

A) amount of nitrogen or phosphorus added to the pitcher of each plant
B) number of insects fed to each plant
C) ratio of the pitcher opening to pitcher length
D) amount of nitrogen or phosphorus added to the roots of each plant
E) ratio of the keel width to total width