What is a groin vault? a. A groin vault is a structure that resembles the top half of a sphere. B. A groin vault comes from the extension of a round arch, similar to a long tube of arches. C. A groin vault has two barrel vaults of the same diameter which cross or intersect one another. D. A groin vault has two barrels which extrude into a two dimensional structure. Please select the best answer from the choices provided A B C D.