A Retail Hardware Chain Is Considering a Self-Service System for Its

A retail hardware chain is considering a self-service system for its 326 stores. Ten stores have been selected for a trial period of three months. Ten other stores, matching the experimental stores as closely as possible, have been selected as a control group in which the traditional clerk system will operate. Dollar sales for each of the twenty stores will be recorded weekly for the three month period. Prior to the study, weekly sales will be recorded for a month, as a "before" measure. The difference in sales between the experimental and control groups is expected to help management in its decision.
Answer the following three questions on the basis of the above information.
-Select the alternative that does not represent an extraneous variable.

A) method of operation (self-service vs. clerk)
B) differences in items offered in self-service vs. clerk operated stores
C) any special promotion by any store or stores
D) time period selected, e.g., near Christmas time
E) differences in physical attractiveness of the stores