Which of the Following Would Be Considered an Example of "Viral

Which of the following would be considered an example of "viral marketing"?

A) elementary students sending emails to their pen pals in another country
B) high school students creating a PowerPoint to share with other students in their high school
C) university students creating a discussion board for other university students to discuss course projects
D) social media site providing an opportunity for protestors to share their point of view