Question: Valhalla is a hamlet and census-designated place (CDP) located within the town of Mount Pleasant, in Westchester County, is a county in the U.S. state of New York, it is the second-most populous county on the mainland of New York, after which location?
Sentence (from "Westchester County, New York"): The county seat is the city of White Plains, while the most populous municipality in the county is the city of Yonkers, with an estimated 200,807 residents in 2016.
Relevant evidence: No