411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...c6/2. d4/2...d5/3. e5/3...c5/4. c3/4...Nc6/5. Nf3/5...cxd4/6. cxd4/6...Bg4
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Caro-Kann - Advance Variation, Botvinnik-Carls
6... Bg4 · Caro-Kann - Botvinnik-Carls Defence
Black is threatening to remove the knight defending d4, and White would need to either remove the queen from its defence, or double their pawns. They can react with the more aggressive 7. Bb5, play passively with 7. Be2, use the dark squared bishop to defend, with 7. Be3, or counterattack on the d5 pawn with 7. Nc3. 7. Nbd2 also defends the pawn, as if Black is lured into taking the pawn with 7...Nxd4, White can simply win the knight with 8. Nxd4. White cannot capture the queen without losing their own queen to 9. Bb5+.
Theory table
7. Bb5 7. Be2 7. Be3 7. Nc3 7. Nbd2
References
Caro-Kann