Given the task definition and input, reply with output. 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.

Castiel: It 's Enokian . I believe it 's some sort of riddle . Why is six afraid of seven ? Now ... I assume it 's because ... seven is a prime number . And prime numbers can be intimidating .
Sam Winchester: It 's because seven ate _ nine _ .
Castiel: It 's word play . And the answer is the key like the Doors of Durin in Lord of the Rings .
Sam Winchester: Wait a second . You know about The Lord of the Rings ?
Castiel: I 'm very pop culture savvy now .
number