As a general statement, a foul occurs when a player does something wrong that puts the other team at a disadvantage.
In the play presented, A1 was put at a disadvantage, but B1 did nothing wrong -- so it's not a foul on B1.
B1 wasn't put at a disadvantage, so it's not a foul on A1.
If A1 was dribbling, then it's not travelling.
That leaves a "no call".
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