I get Hwayoung injury but she was at the rehearsal && decided to leave all the sudden. Sitting on the chair singing won't damage your image && isn't hard it's kinda easy,you don't have to dance. Plus if she didn't want to do it she could have said so earlier so they could have practiced earlier && during rehearsal. They didn't have Soyeon there either who sings lots of lines as well so having to remember Soyeon and Hwayoung lines will be kinda tough.