How did Armin become a Titan?

Related questions

1. How many seasons are there to attack on Titan?
2. Did Armin Die in Attack on Titan?
3. Is there a season 4 of Attack on Titan?
4. Are Eren and Mikasa related?
5. Are Mikasa and Eren siblings?
6. Is Attack on Titan Season 3 over?
7. Are Mikasa and Eren in love?
8. Is Mikasa adopted?
9. What Titan powers does Eren have?
10. Is Mikasa and Levi cousins?

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19. #titans
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21. #serum
22. #obtained