Answer
a. a compensating differential
Work Step by Step
Compensating differential is the concept of explaining wage differences in terms of non monetary characteristics of the job like the ease of working, conditions of the work environment, efforts involved in the job etc. In the case of Lucy and Ethel, Lucy does the job if greeting customers which is easy, fun and less straining compared to the job of Ethel who cleans the bathrooms which is hard, dull and involves a filthy work environment.