open spanish - 9. c3 vs. 9. Nbd2

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thebird

In the open spanish (1. e4 e5 2. Nf3 Nf6 3. Bb4 a6 4. Ba4 Nf6 5. 0-0 Nxe4 6. d4 b5 7. Bb3 d5 8. dxe5 Be6)  it seems like grandmasters these days all play 9. Nbd2 rather than 9. c3.

Has someone found a problem with 9. c3? Or is 9. Nbd2 so much better?