Kevin and Cheryl had completed their first day of naturalistic observation of adolescents at the local high school. As they discussed their experiences during the day, they found that they were both concerned about the difference their presence in the classrooms made on adolescent behavior. This is an important question since, in truly naturalistic research, researchers should:

A. be unobtrusive   
B. manipulate the independent variable   
C. be seen and interact with the subjects   
D. not change the situation   
E. operationally define the variables   
F. record the observed variables that are part of the study

Apply as many.