The Organization's Retention of Knowledge Within Itself Is Threatened by Retirees

The organization's retention of knowledge within itself is threatened by retirees taking all of their skills and knowledge with them when they leave.All of the following are potential solutions to that problem EXCEPT

A) having potential retirees record "how to" videos.
B) keeping former employees on call for a period after they leave.
C) requiring critical employees to train and mentor others.
D) restructuring the work in the organization so that no single employee has unique knowledge.