Q: 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.
Michael Vaughn: Now , when you and Sark meet , you will actually be able to test each other 's merchandise , which is why we need to give him the real Rambaldi solution . After he confirms that the liquid is authentic , you 'll make the deal , then switch the vial with an identical _ one _ containing colored water and a radioactive isotope we can track via satellite .
A:
vial