Lupe is conducting an experiment to find out what type of antibacterial soap kills the most bacteria. In her experiment, Lupe grows E. Coli bacteria in ten different Petri dishes. Lupe then sprays 10 milliliters of an antibacterial soap on nine of the Petri dishes. Each of the nine Petri dishes is sprayed with a different brand of soap. Lupe doesn't spray any soap on the tenth Petri dish. In Lupe's experiment, what is the test variable (independent variable)