Allusion is a figure of speech, in which an object or circumstance from unrelated context is referenced covertly or indirectly. It is left to the audience to make the direct connection; where the connection is directly and explicitly stated by the author, it is instead usually termed a reference
1. Is number 2
2. Is number 3
3. Is number 1
4. Is 3
5. Is 1
6. Is 2