In the Kaplan-Meier graph, why are the estimated curves for both groups (Treatment and Placebo) at 1 (100%) when time since randomization (the follow-up period) is 0 months? a. Because 100% of the subjects relapsed at 0 months of follow-up. b. Because 100% of the subjects were censored by the end of the study. c. Because the follow-up period is less than 2 years. d. Because all subjects followed in this study were event free (had not yet relapsed) at the beginning of the study