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Fellow member D.ana tweeted on December 9, "My same-aged friend Nahyun from our group SONAMOO, who are preparing for a new beginning, has celebrated her 20th birthday!! Everyone, please give a lot of congratulatory messages for Nahyun's birthday~^^/Love/ Thank you, everyone<3," along with a snapshot from the surprise birthday party.

The green backdrop also ties in with the mysterious title "GREEN MOON", which was previously revealed on their Facebook.

Happy Birthday, Nahyun!