b1a4 'first official appearance as a trio after leaving the camp'

KBS 2TV Music Bank' rehearsal was held at the public hall of KBS New Pavilion in Yeouido, Yeongdeungpo-gu, Seoul on December 21st.

On that day, b1a4 (Sandle Ballroom Shin Woo) attended the rehearsal.

About Music Bank

Music Bank is a South Korean music program which airs every Friday at 17:00 KST on KBS2. As of 2015, the show is also broadcast in more than a hundred countries through KBS World.  Wikipedia