why did hannah take eva's place in fhe death chamber?

I'm pretty sure the movie would be similar as this is the climax of the story. The guard chooses all the girls except Hannah to be taken away to be exterminated. Hannah then makes the supreme sacrifice to save her friend Rivka by tricking the guard into taking her instead. This great sacrifice, greater because it was not asked for, saves Rivka's life and returns Hannah to her family. Her faith is rewarded and she is reunited with her family and her own life while Rivka is able to live on.