The Employee Health Service has notified the IP that seven employees have P. aeruginosa folliculitis. Initial investigation reveals that six of the seven cases belong to the same health club. Working on the hypothesis that the whirlpool at the health club is associated with the infections, the IP decides to conduct
a case-control study using two controls for each case. Which of the following groups is the most appropriate control?

a. Non-ill family members of the ill employees
b. Non-ill hospital employees matched for age and sex
c. Hospitalized patients with P. aeruginosa folliculitis matched for age and sex
d. Non-ill members of the health club matched for age and sex