How does black trap a piece? Puzzle!

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laurengoodkindchess

Hi! My name is Lauren Goodkind and I’m a chess coach and chess YouTuber based in California: 

https://www.youtube.com/channel/UCP5SPSG_sWSYPjqJYMNwL_Q 

How does black trap a piece? This position was taken from a game that I played today.  

hac817

b4

dikmasterson

Which is the best follow-up move for black if white bishop captures b4 black pawn?

Nb4, cb4 or ab4?

laurengoodkindchess

b4 is correct!