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
개방계는 다양한 정보와 자원을 공유하는 시스템입니다.gaebang-gye-neun dayanghan jeongbo-wa ja-won-eul gong-yuhaneun siseutem-imnida.An open system is a system that shares various information and resources.