What did Janice Kiecolt-Glaser's classic study of Ohio State University medical students find regarding the impact of stress on their immune system?
a. An increase in the number of lymphocytes during exams.
b. No significant change in lymphocyte levels before, during, or after exams.
c. A decrease in the number of lymphocytes during the first day of exams.
d. A decrease in the number of lymphocytes after the exam period.