For which of the following pairs of variables would it be appropriate to calculate the correlation coefficient ($r$)?

A. Political party affiliation of STAT 200 students and whether they voted in the previous Alma Mater Society (AMS) elections.
B. Cell phone brand and monthly cell phone charges for phones used by STAT 200 students.
C. Income of student and the name of the company that employs them, for STAT 200 students who work part time.
D. Time spent studying for the final exam and score on the exam for STAT 200 students.