Definition: In this task, you will use your knowledge about language (and common sense) to determine what element the marked number refers to. The numbers are marked with two underlines around them, like: _ number _. Your answer should be chosen from the given text, and should not contain other words.
Input: Kate Beckett:  Something 's not right here . How did Peter reach the vent from inside the cell ?
Kate Beckett, Richard Castle: It was a two man job !
Richard Castle: Two prisoners escaped , but only _ one _ was missing . Which means ...
Kate Beckett, Richard Castle: The killer came back .

prisoners