In the "Power" Approach to Defining Professions, What Makes the Knowledge

In the "power" approach to defining professions, what makes the knowledge held by professionals unique?

A) Although the knowledge is often held by general public, the professional has power over the client.
B) Professional knowledge is both theoretical and applied.
C) Professionals use this knowledge in doing pro bono work.
D) The knowledge is monopolized.
E) The knowledge is conveyed in formal schooling.