In a retrospective study of 1,000 patients with unilateral acoustic neuromas, a team of researchers reviewed medical charts and entered data on every possible risk factor into a massive database. 1,000 demographically-matched control patients were randomly selected from the national population. A machine learning program was then used to analyze the data. These researchers found that compared to the controls, patients with unilateral acoustic neuromas spent, on average, 261% more minutes on their cell phone each day. Further analysis revealed that heavy cell phone use represented a relative risk of 17.6 (p=0.001) for the development of an acoustic neuroma. Which of the following most accurately describes the error in this study?

A. The study suffered from selection bias
B. The study was underpowered
C. The study used a machine learning program
D. Risk measure selection was inappropriate
E. The study was poorly designed