Selma wants to find out at which temperature Streptococcus bacteria reproduce the fastest. Selma places the same size sample of the bacteria on 10 different Petri dishes filled with nutrient agar. She then places each dish in different incubators which are all set at different temperatures. After three hours, Selma removes the dishes from the incubators and observes the size of the bacterial colonies. What is the variable in Selma's experiment?