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"): Situated in the Hudson Valley, Westchester covers an area of 450 sqmi , consisting of 6 cities, 19 towns, and 23 villages.
Relevant evidence: No