Jake and Jack Are Both Interesting in the Effect of the Loud

Jake and Jack are both interesting in the effect of the loud music from the dorm room next door on their grades.Jake records the days and hours the music is loud and compares that information to the grades he received on exams taken the days following the loud music.Jack plays his own music loud before his first psychology exam and quietly before his second exam.He then compares his exam scores.The primary difference in the way Jake and Jack conducted their research is:

A) Jack only looked at psychology exams and Jake used several courses.
B) Jack had fewer types of data than did Jake.
C) Jake took more accurate measurements than Jack.
D) Jake used correlational data and Jack manipulated a variable.