What is the dependent variable? What independent variable was tested? What unidentified independent variables might have influenced the results?

Read each situation below and answer the questions that follow.
Tom and Juanita conducted an experiment to test the effect of age on difference threshold in children 5 years of age and younger. They created an experimental setup that included cards mounted with 12 pairs of paint chips of the same hue but of various shades. They tested 3 children from each of these age groups: 2, 2.5, 3, 3.5, 4, 4.5, 5.
What is the dependent variable?
What independent variable was tested?
What unidentified independent variables might have influenced the results?