Dr Kulich Is Conducting a Study to See If Scary

Dr. Kulich is conducting a study to see if scary ghost stories cause fear in young children. She recruits 50 children to be in her study and she tells half of the children a scary ghost story. The other half are told a happy story about baby farm animals. She measures the heart rate of the children as an indication of fear. In this study, heart rate is:

A) negatively correlated with fear.
B) an independent variable.
C) an operational definition of fear.
D) a confounded variable.