Arthur is testing how well various types of disinfectants can kill E. coli bacteria. He starts with a Petri dish that is covered with E. coli. He puts exactly one milliliter of each different disinfectant at a different location on the Petri dish. If a disinfectant works, it will eliminate the bacteria in the area in which it was placed. Two days later, Arthur will measure the amount of bacteria that has been cleared out by each disinfectant. Which of the following is a variable in this experiment? A. the type of disinfectant used B. the type of bacteria used C. the Petri dish used D. the length of time that the disinfectant is used