In the Section "Investigating Life: the Hidden Cost of Traps",what

In the section "Investigating Life: The Hidden Cost of Traps",what was the independent variable?

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