A psychologist studied individuals' responses to different types of music. Students were assigned to one of two groups. Group A listened to loud alternative rock music (e.g., Tool) for five minutes while he measured their heart rate in beats per minute. Group B listened to loud country music (e.g., Luke Bryan) for five minutes while he measured their heart rate in beats per minute.

On what scale of measurement is the dependent variable measured?

a. ordinal

b. ratio

c. nominal

d. interval