Which of the Following Represents an Example of Social Facilitation

Which of the following represents an example of social facilitation?

A) A talented musician performs better in front of a instructor than an audience.
B) An individual's test score improves after studying with a group of friends the previous evening.
C) An individual concentrates better on his exam after the teacher moves him to a more quiet room.
D) An individual runs faster with a group of friends than by herself.