Black Friday: lifetime access is on a special offer in the app, until November 30. Black Friday: special offer on lifetime access, until November 30. Download
철도 궤간은 기차의 안전한 운행을 위해 중요하다.cheoldo gwegan-eun gicha-ui anjeonhan unhaeng-eul wihae jungyohada.The railway gauge is important for the safe operation of trains.