Dr.Rosenfeld plans to study the relationship between people's smoking behaviour and their tendency to have minor physical illnesses (such as colds or the flu) .Why might she be most likely to choose correlational research for this study

A)  Correlational studies allow the researcher to draw strong cause-and-effect conclusions. 
B)  The university does not allow smoking in the psychology building. 
C)  It is not practical or ethical to manipulate people's smoking behaviour. 
D)  Correlational studies are always the "first choice" of researchers.