17. At any rate, the separation ratio is accurate. 无论如何,这个分离比是精确的. 18. The boundary around the round ground separates us from the surroundings. 围绕着圆形场地的边界将我们同四周隔开. 19. The blunder made the underground instrument undergo an undermining of the thunderbolt. 这个失策让地下仪器经受了一次雷电的破坏。