Concept:Wage differences are explained by economic factors like training and market forces, not by job categories such as teaching or non-teaching.Explanation:A principal undergoes longer and more costly training, which increases skill and productivity.Market demand and supply also influence the salary difference between principals and messengers.Being a non-teaching staff member is not an economic reason for earning a lower salary.Hence, that option fails to explain the wage gap.Answer:D. the messenger is a non-teaching staff