Based on the following excerpt, identify the most likely study design. In this study, we examined the effects of supplemental vitamin D3 as compared with placebo on incident fractures in 25,871 U.S. men (age, 50 years) and women (age, 55 years), including 5106 Black participants, who were enrolled from all 50 states and followed for a median of 5.3 years. Case-control Cross-sectional Qualitative study Randomized-controlled trial Systematic review