Which of the following statements about retirement benefits under pension plans is true?

a. Under a flat percentage of annual earnings defined benefit formula, each employee receives the same dollar benefit.

b. A benefit using final pay is usually based on an employee's earnings during the last month of plan participation.

c. A unit-benefit formula considers both earnings and years of service.

d. Past service benefits are the result of bonuses and overtime pay during the period an employee participated in the plan.