An observational study conducted in inner-city Boston approximately four years ago demonstrated that high-school students who consumed more than five cans of non-diet soda every week were between 9 and 15-percent likelier to engage in an aggressive act compared with counterparts who drank less non-diet soda. Based on this evidence, we can reliably conclude that .

C. a third variable, such as students' home environment, must have caused students to both drink more non-diet soda and engage in aggressive acts  

Explanation:

It is not possible to say that there is a direct relationship between the consumption of non-diet soda and involvement in violent situations, since the study does not show the same evaluation in adolescents who do not use this type of drink. Thus, it is more efficient to consider as a third variable, the home environment in which these students live and the relationship between that environment, the consumption of non-diet soda and involvement in violent acts.