A Placebo Effect Makes It Difficult to Draw Cause and Effect

A placebo effect makes it difficult to draw cause and effect conclusions because we cannot tell whether

A) the variables were operationally defined.
B) random sampling or random assignment is appropriate for the study.
C) it is the independent variable or the dependent variable that are responsible for the results.
D) it is the treatment or the participant's expectations that are responsible for the results.
E) it is the treatment or the researcher's behaviors that are responsible for the results.