Answer (a) — incorrect: Subjects were not randomized in this study. Subjects can only be randomized in experimental designs (i.e., RCT's). The behavior of subjects cannot be manipulated by investigators conducting observational studies (e.g., cohort or case-control studies)

Answer (b) — incorrect: In this study cases and controls were not restricted to any specific age category.

Answer (c) — correct: Controls were frequency matched to cases by age (in 5-year intervals). See Ashengrau & Seage, pp. 295-296 for details on frequency matching. Matching in a case-control study is intended to created comparability in the underlying source population by creating "mini-studies" in which all individuals are within the same specific 5-year age range such that age can have no effect on the exposure disease relation in that particular mini-study.