Kyuhyun and other MCs think Kim Gura got irritated because he didn't know the meaning of "Best Friend" (KyuHyun even shortens the word Best Friend *베스트프렌드 to 베프 which is Korean slang for best friends) but it looks like Kim Gura knew because he turned to the MC on the other side and said 알아 (Al-a which means "I know").