Fifty undergraduate and twenty graduate student volunteers participated in a study of whether ambient temperature affects cognition. Twenty-five undergraduate and ten graduate students were randomly assigned to a 60° room; the others were assigned to an 85° room. Each group was asked to memorize a list of nonsense words. The researchers compared the average number of words memorized for the two groups. What type of variable is student classification (graduate or undergraduate)?

a. Treatment variable
b. Blocking variable
c. Explanatory variable
d. Lurking variable
e. Response variable