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: David Zimmerman:  How come I 've never heard of this guy ?
Derek Morgan: Have you ever heard of Efren Saldivar ?
David Zimmerman: No .
Derek Morgan: David Parker Ray ? How about John Edward Robinson ? See , at any given time , we have at least 25 open cases . Every year we 're able to close around _ 15 _ , yet new ones still seem to pop up . There are more serial killers out there than you may think .

cases