At OB Inc., you are charged with recruiting employees in your department for a new project team. You begin by asking Sam to be a part of your team. You and Sam have been friends for years and have much in common, so you feel certain he will agree to be a part of the team when you ask him. The persuasion principle that applies most appropriately to this scenario is:

a. reciprocity.
b. authority.
c. liking.
d. social proof.
e. consistency.