What very difficult sorting decision does Cassia make regarding Ky and his workplace?

Cassia is faced with a challenge greater than any challenge she has face in her life. She must sort the workers in Ky's work area. Not only does she have to block out the horrors of the actual workplace, and the mental horror as she learns of poisoning, but she has to make decisions on who is going to have the opportunity to leave the place although their destination is unknown. She chooses what she hopes is best for Ky, who is right between the groups, just as he always was when they played games. He is in the group to leave, even if it means leaving her. The decision is horrible, but she thinks she is saving his life.