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
회전원통은 비행기의 엔진에서 중요한 역할을 합니다.hoejeonwontong-eun bihaenggi-ui enjin-eseo jungyohan yeoghal-eul hamnida.The rotor plays an important role in the engine of an airplane.