Based on the following excerpt, identify the most likely study design. The data used in this article were drawn primarily from more than 30 months of fieldwork, spanning 2018 to 2021, conducted in Tijuana, Mexico, by 2 of the study authors. Fieldwork was initially based at harm-reduction sites and expanded naturally through snowball sampling to significant locations frequented by participants. Case-control Cross-sectional Qualitative Randomized-controlled trial Systematic review