A countermelody is an addition to the main melody, creating a harmony. So D (:
at least one is to show emotion
Answer
D.Experimental bias
Explanations
Experimental bias occurs when a scientist performing a research influence the results in order to portray a certain outcome.In this case,Ron unintentionally smiled when then subject was showing to act as expected.In doing so,"smiling" the subject was able to identify the correct answer.
