There were 2,047 children randomized to the placebo group, and 662 still at risk of contracting rotavirus gastroenteritis after 60 weeks of follow-up. In other words, 32% of the placebo sample was still being followed in the study and at risk of contracting rotavirus gastroenteritis at 60 weeks. The corresponding Kaplan-Meier curve estimate of the percentage of children in the placebo group who survived beyond 60 weeks without developing rotavirus gastroenteritis is approximately 93%. How can this have happened?

a. There was very little censoring, if any, prior to 60 weeks among the observations in the placebo group.

b. The researchers dropped all of the censored observations in the placebo sample prior to estimating the given Kaplan-Meier curve.

c. Some of the observations in the placebo group were censored before 60 weeks (lost to follow-up)

d. The Kaplan-Meier curve estimate at 60 weeks is the risk of rotavirus gastroenteritis for those who had still not contracted rotavirus gastroenteritis and were still enrolled in the study at 60 weeks.