Rather Than Create the Levels of the Independent Variable Through

Rather than create the levels of the independent variable through manipulation, researchers can select people who fall into groups the researchers want to contrast.

A) This latter type of research is an ex post facto design rather than an experimental design.
B) The researchers must control other variables, or they will not be able to make clear interpretations of causality in this nonexperimental research.
C) The researchers should consider a survey rather than the design they had planned.
D) The researchers should include another independent variable to add contrasts.