Acase-control study was conducted to assess whether occupational radiation exposure among men was associated with down syndrome in their children. the investigators matched cases and controls on age of the mother at childbirth by 5-year categories. why was this done?

(a) to minimize recall bias of exposure
(b) to minimize detection bias for cases
(c) to minimize differential misclassification of exposure
(d) to remove selection bias of non-radiation exposed fathers
(e) to provide dose-response information