In a park, a guy you do not know sits near you. His mere presence affects your behavior even though he is doing nothing. In the context of social facilitation, this person is best described as a(n) .

Co-actor

Explanation:

Co-star or co-actors or actresses are those individuals who a role in the movie with the main actor or actresses. They play a very important role in the movie or serials. When the main lead actor and actresses are co adjusted with co actresses, they even play a great role in the film.  

Thus in the above context, a guy whom you don't know and sit nearby you. The mere presence of that person has affected your behavior, yet he is doing nothing. In this context, that person plays the role of co-actor.

answer; ///first they would cross the atlantic ocean, and end up either in europe or africa; good luck