Professor Li Finds That High School Students Who Spend More

Professor Li finds that high school students who spend more time on social media have higher levels of depression. This difference is statistically significant. Assuming that the measures had adequate validity and that proper research procedures were followed, it would be most appropriate to conclude that, for this population,

A) spending time on social media increases depression.
B) being more depressed causes students to spend more time on social media.
C) some unmeasured third variable causes both higher social media use and increased depression.
D) any of the above could be true.