A researcher evaluates the effectiveness of an art class by having an art expert judge the quality of student’s paintings at the beginning of the class, in the middle of the class, and at the end of...

A researcher evaluates the effectiveness of an art class by having an art expert judge the quality of student’s paintings at the beginning of the class, in the middle of the class, and at the end of the class. If the art expert’s standards for judgment change during the class, then the internal validity of the study is threatened by

a. an extraneous variable.

b. individual differences.

 c. a time-related variable.

d. an environmental variable.