British investigators conducted a study to examine measles- mumps-rubella (mmr) vaccine history among 1,294 children with pervasive development disorder (e. g., autism and asperger's syndrome) and 4,469 children without such disorders. (they found no association.) this is an example of which type(s) of study?
a. randomize control trial
b. case-control
c. cross-sectional
d. retrospective cohort

B. Case-Control

Explanation:

A case-control is a study that compares a group with an outcome of interest (cases) with a group who do not have the outcome (controls). This study aims to looks back retrospectively to compare how frequently the exposure to a risk factor is present in each group and with that to determine the relationship between that risk factor and the outcome.

So in this case, the cases are children with pervasive development disorder and the control is those without the disorders.

These studies are normally observational because no intervention is attempted.