Why do union contracts protect workers who have seniority?

a. older workers have more voting power in unions; they vote to protect themselves.
b. unions tend to support older members because they pay more in union dues.
c. businesses are more likely to fire older, higher-paid workers. seniority rules protect older workers.
d. united states law forces unions to consider seniority in collective bargaining

C.  businesses are likely to fire older workers.  They are paid higher, due to working many years.  Its cheaper to higher new young people at a much lower pay to save money for the company.  Older people may not have went to college, but have learned the job by doing it for so many years.  Companys now want to have a college degree to work for them.  The union protects the older worker, so the company can not suddenly change the rules and force the worker out.  They have to offer to let the older worker go back to school to earn the degree required.