Anthony Heard a Report About a Study in the Journal

Anthony heard a report about a study in the Journal of the American Medical Association that stated working night shifts causes heart attacks. When he read the study he found that the researchers had compared the rates of heart attacks between day and night shift workers. This was not an experimental design and subjects were not randomly assigned. If Anthony thinks critically about this study he would conclude that _____

A) this is good evidence that night shift work causes heart attacks.
B) this proves it is unhealthy to work night shifts.
C) this study shows a correlation, proving causality, between night shifts and heart attacks.
D) this type of study can't provide evidence of causes.