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.
Lucas Friar: Hey , I found the fourth key !
Riley Matthews: Very impressive , Lucas ! How 'd you know it was in there ?
Lucas Friar: Well , it was the only _ one _ not glued to the shelf . Plus , it 's called ' The Fourth Key ' . And all the other ones are called ' Not This One ' .
Riley Matthews: ... a little less impressive .
A:
key