Teacher: 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 _. There are several possible answers, you'll need to choose the proper one. Carefully read the given text, pay special attention to the marked number, think about what (unwritten) information the marked number holds inside, choose the most adequate word(s) from the optional answers. If none of them seems right to you, there's also an option for other. If your answer is "REFERENCE", also write the reference entity, otherwise write the implicit option name. Options to choose from are:
REFERENCE: Some object which is being mentioned in the text before or after the target number. The reference answer has a higher priority than any other. If both Reference and another answer are possible, prioritize the Reference.
YEAR: Describing a calendric year
AGE: Describing someone's age
CURRENCY: Reference to some monetary value e.g dollar, euro etc.
PEOPLE: Describing a single/plural persons
TIME: Describing a time of the day. Usually you can add the word o'clock after those numbers.
OTHER: Some other option, which isn't listed here.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Jess Mastriani: No, I don't want another crooler, thank you very much.
 FBI Agent Nicole Scott: But it's good for you. It's got... honeyglaze. Please die for this crooler, Jess.
 Jess Mastriani: I've had _ two _ already. Who eats three croolers in a night? 
FBI Agent Nicole Scott: Take a look. [Nicole takes a huge bite] Mmmmm, Mmmmm, Mmmmm!
Solution: REFERENCE crooler
Reason: In this example, the number two refers to something that appears in this text. In this example, it refers to the word: crooler.

Now, solve this instance: 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 .
Student:
REFERENCE cells