Why Have Seniority- Based Provisions Become More Controversial from an Efficiency

Why have seniority- based provisions become more controversial from an efficiency perspective?

A) because there are many fewer interchangeable jobs than there used to be
B) because older workers are less capable than younger workers
C) because most workers are staying on until an older age these days
D) because these provisions are more difficult to administer than they used to be
E) because youth unemployment is on the increase