Selena was inside the building (house) sewing cloth into the shape of an "O". That was the "O" that would go at the end of the word H-E-L-L-"O" that would signal to the planes that someone was down there and still alive.
It might also be symbolic that the word "hell" was visible. Meaning that they had been living in a virtual hell. By adding the "O" they could turn hell into rescue.
They had been saying "Hello" all throughout the movie. It was usually when they would say "hello" that an infected would attack.
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