Eva is performing an experiment to determine which type of disinfectant kills the most bacteria. She has six plates of the same kind and amount of bacteria. She adds one of five different kinds of disinfectant to each plate and leaves one plate without any disinfectant as a control.

What role does the test variable (independent variable) play in this experiment?
A.
The type of disinfectant determines the amount of bacteria killed.
B.
The type of disinfectant used determines the type of bacteria used.
C.
The amount of bacteria killed determines the type of disinfectant used.
D.
The amount of bacteria killed determines the type of bacteria used.