Answer:
c. can be valid if they are not used to perpetuate past discrimination.
Step-by-step explanation:
Seniority system is a system prevalent at a workplace that gives preference to the employees according to the length of service provided (seniority) o the company. The system provides certain kinds of benefits like reward, promotion or job security based on the length of the service of the employee. It is not illegal because it does not prevent someone to achieve benefits but it gave first preference to those who were serving the longest with the firm to achieve those benefits. Therefore it is different from age discrimination based on one's age.