Which statement best describes social media as a platform for marketing?

a$.$ No metrics have been developed to measure social media's value.

b$.$ The content on social media flows one way, from brands to consumers.

c$.$ Most marketers have not been able to figure out how to measure the benefits of social media.

d$.$ Social media have drastically changed the advertising business from an industry based on relationships and conversations to an industry based on mass$-$media models.