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: Eric Carter:  Rebecca , it 's Carter . Ben got away . Please tell me you 're still tracking him .
Rebecca Ingram: We were . He left the construction site heading east , but we lost him on 13th .
Eric Carter: I found out why Bin - Khalid 's people wanted the strongbox . Inside there was a flash drive containing a list of sleeper cells that Bin - Khalid set up here , along with all the codes to trigger their attacks . It looked like _ fifteen _ , maybe twenty cells , but I did n't have the list for very long . He said he 'll sell it to the terrorists if we do n't give him what he wants . I 'm still waiting to find out what that is .
Rebecca Ingram: Do you believe he 'd really do that ?
Eric Carter: He 's angry and he 's unstable . I do .

cells