If We Want to Study the Effect of Social Media

If we want to study the effect of social media on the GPAs of college freshmen, the best study is:

A) a survey to be conducted online at the end of freshman year, in which we ask students how much time they spent on sites such as Facebook and we record their GPAs.
B) an observational study that compares the GPAs of a group of freshmen who were observed to have spent at least three hours per day on social media to those who spent less than 30 minutes per day.
C) a study that recruits volunteers who agreed to spend at least two hours per day on social media and another group of volunteers who agreed to spend no more than 30 minutes per day on social media.
D) None of the answer options is correct.