Please answer the following question: Kidd's Mills Covered Bridge Historic District is a national historic district located at Pymatuning Township Mercer County Pennsylvania. The district includes two contributing sites and one contributing structure. The Kidd's Mills Covered Bridge is a historic wooden covered bridge built in 1868. It is rare example of a Smith Cross Truss bridge in the eastern United States. The district includes the sites of two grist mills.It was added to the National Register of Historic Places in 1974. Given a list of categories: company, educational institution, artist, athlete, office holder, mean of transportation, building, natural place, village, animal, plant, album, film or written work, what category does the paragraph belong to?
A:
Building