What is the premise of "two-tier" wage plans in a unionized environment?

What is the premise of "two-tier" wage plans in a unionized environment?

Question 31 options:

a) 

Workers on different shifts receive shift differentials

b) 

Union and nonunion workers are paid two different rates

c) 

Workers hired after a specified date will receive lower wages than more senior workers

d) 

Management and union employees are paid two different rates

e) 

After a certain number of years on the job, a worker can be promoted