What is the best continuation for Black?

Indianapolis open, Chess.com
Analysis of Xun (1400) vs. Sims (1558)

1 2 3 4 5 6 7 8 h g f e d c b a 0

(black to move) 1. c4 e5 2. Nc3 Nf6 3. g3 c6 4. d4 e4 5. Bg2 d5 6. b3 Bb4 7. Bd2 e3 8. fxe3 Ng4 9. Qc2 Qe7 10. a3 Bxc3 11. Qxc3 O-O 12. h3 Nf6 13. O-O-O Qxa3+ 14. Qb2 Qd6 15. g4 a5 16. Nf3 Nbd7 17. Ne1 Re8 18. Nd3 a4 19. Rhf1 axb3 20. Qxb3 dxc4 21. Qxc4 Qa3+

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