A student is investigating the effectiveness of three different household disinfectants against Escherichia coli bacteria. They prepared nutrient agar plates inoculated with E. coli and placed filter paper discs soaked in each disinfectant onto the agar.
All the plates were incubated at 25∘C25^\circ\text{C}25∘C.
State two variables, other than temperature, that the student must control to ensure a fair test.
