This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
A local restaurant was the focus of a botulism outbreak.
The state epidemiologist most likely used which format to determine the cause of the disease? 
Select and explain. 
A. Cross sectional study, B. Prospective study, C. Retrospective study, D. Case–control study
C: Retrospective study
Answer: C: Retrospective study 
Explanation: Cross-sectional studies (Prevalence studies) 
The simplest in concept and execution is the cross-sectional study. In this, a set of individuals are chosen who may be a representative sample of the general population or of people in a particular community or a sample of members of some special subgroup - school children, the armed forces, workers in a particular industry, etc. The measurements of exposure and effect are made at the same time providing information on the relationship between a disease and other variables of interest, as they exist at one point of time. Cross-sectional studies are relatively easy and economical to carry out. 
Reference: Essentials of Preventive and Community Dentisitry, Soben Peter , 4th ed page no 57