Jerome is performing an experiment for the science fair. he wants to know how long it takes for water to evaporate at varying temperatures. so, he pours exactly 1 ml of water into each of five identically-sized test tubes. then, he places the test tubes in five different incubators which are set at 20°c, 30°c, 40°c, 50°c, and 60°c. finally, he measures the amount of time it takes for the water to completely evaporate at each temperature. what is the independent variable in jerome's experiment?