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
이 반동작용은 힘의 균형을 유지하는 데 중요하다.i bandongjak-yong-eun him-ui gyunhyeong-eul yujihaneun de jungyohada.This counteraction is important for maintaining the balance of forces.