Saffi, one of the popular schools in San Francisco did a school wide study of the students in middle school. The study found that there is a negative correlation between the time spent on Facebook per day by students and their academic achievement. How can we understand the results of this study?
A: An increase in time spent on Facebook per day causes a drop in the academic achievement of students at the middle school level.

B: There is an association between an increase in time spent on Facebook per day and the drop in the academic achievement of students at Saffi. 

C: An increase in the time spent on Facebook per day causes a drop in the academic achievement of students at Saffi. 

D: There is an association between an increase in time spent on Facebook per day and the drop in the academic achievement of students at the middle school level.