Answer: c. a compensating wage differential
Explanation: since different jobs in the labour market are characterised by different wages, workers do have different preferences as do firms who employ these workers have different conditions wherein they work. Soft coal miners earned a higher hourly wage than other industrial workers of similar skill levels due to compensating wage differentials. This is the additional amount of income that a worker must be offered in order to motivate them to accept a given job (often undesirable and collectively known as nonwage characteristics of the job), relative to other jobs that the same worker could perform.
For unpleasant jobs, firms often offer higher wages in order to attract workers. The reverse is also true.
