U 31 was a class of U-boats built during World War I by the Kaiserliche Marine.U 31 U-boats carried 6 torpedoes and were originally armed with one 8.8 centimetres (3.5 in) deck gun which was replaced in 1916/17 by a 10.5 centimetres (4.1 in) deck gun. 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:
Mean Of Transportation