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5.c3 Qb6: Qc2, not Qb3

When you play 5.c3 instead of 5.Nbd2, Black answers 5…Qb6 (8 games, 25%). You follow with 6.Qb3 (5 games, 20%), then 6…c4 7.Qxb6 axb6: 4 games, 0%.

Recall: why is b2 weak in the London?