Which of the Following Encounters Exemplifies Interpersonal Communication

Which of the following encounters exemplifies interpersonal communication?

A) You buy a coffee on your way to your morning class and say "thank you" to the barista as he hands you your drink.
B) During class, you discuss course concepts with a group of other students you just met.
C) After class, you share with your roommate your concerns about some upcoming assignments because you know he will be able to relate to your concerns.
D) Later that night, you post an impassioned and opinionated comment to a news site online.