Subjects were children enrolled in a health maintenance organization. At $2$ months, each child was randomly given one of two types of a new vaccine against rotavirus infection. Parents were called by a nurse two weeks later and asked whether the children had experienced any of a list of side$-$effects, this type of study belongs to: a$.$ Experimental Ob$.$ Observational cohort c$.$ Observational case$-$control. d$.$ Observational cross$-$sectional e$.$ None of the above