If a Researcher Plans to Study Graduate-Level Achievement in All

If a researcher plans to study graduate-level achievement in all students who were educated under the Vermont public school system, in a small town that used both state-mandated texts and enrichment texts of the school board's choosing, the researcher would be using a fairly small sample, bound by geography and time. Which type of validity is decreased by a study like this one?

A) Construct validity
B) Statistical conclusion validity
C) External validity
D) Internal validity