The Effect of Adding Fluoride to a Community's Water Supply

The effect of adding fluoride to a community's water supply was tested by comparing annual records of cavities per patient in local dentists' offices for six years before and four years after the fluoride was addeD .This is an example of which type of research?

A) Interrupted time series
B) Regression discontinuity
C) Propensity score matching
D) Cross-sectional comparison