Item CV030001: In an experiment involving three variables (X, Y, and Z) that could affect the outcome of the experiment, changing X and keeping Y and Z constant allows you find out the effect of X on the outcome of the experiment.
A student thinks that there are three variables (X, Y and Z) that may affect the result of her experiment. She decides to change only variable X, and let variables Y and Z stay the same. What is the student trying to find out?

Distribution of Responses Percent of students responding correctly
 