Your client is a $51-$year$-$old participant in the a cash balance plan. They have been a participant in the plan for the last $22$ years. The company, which has over $10,000$ employees, is having financial difficulty and your client is concerned about the security of their pension. Which of the following is correct?

a$.$ The cash balance plan assets may include up to $25\%$ of XYZ stock.

b$.$ The cash balance plan formula cannot be lowered in the future for current participants, but could be changed for new participants.

c$.$ The cash balance plan and its benefits are fully protected by the PBGC$.$

d$.$ Termination of the plan may affect vesting for some employees, but your client's vesting will not be impacted.