Ongoing defined benefit plan liabilities are difficult to estimate because of many reasons, but not because:

Select one

A$.$ An unknown number of future employees do not work long enough to become "vested" in the plan.

B$.$ An unknown number of "vested" employees will earn benefits on uncertain future salaries.

C$.$ Thir discount rate used to calculate present values is regulated by legal requirements but may not be accurate.

D$.$ The accumulated benefit obligation $($ABO$)$ is subject to changes in vesting rules.