Juan and Sylvester are collaborating on a group project for a class but are having trouble agreeing on objectives and goals. Their professor senses the trouble they are having and steps in to help them refine their goals and come to an agreement. The professor assumed which of Georg Simmel's roles?

A. mediator
B. team leader
C. divide et impera (divide and conquer)
D. tertius gaudens (the third that rejoices)