常見問題：
問：1. Q: What is Secret? 答： A: Secret is a South Korean girl group formed by TS Entertainment in 2009.
問：4. Q: What was Secret's debut song? 答： A: Secret's debut song was "I Want You Back," released in 2009.
問：9. Q: What is the reason behind Song Ji-eun's departure from Secret? 答： A: Song Ji-eun left Secret in 2014 to focus on her solo career and acting.