GMAT Critical Reasoning (CR) | Re: A significant number of Unitron Corporation’s department heads are due

Although D is the best, Only problem I have with D is :

The number of employees other than current department heads who could take on the position of department head is equal to only about half of the expected vacancies. :The number of employees other than current department heads including everybody in lower hierarchy from current department heads and upper heirarchy from Current department head.

unitron can fill the vacancy

A. outside the company
B. current department heads take over