hexagon7: I thought that B C3, W C2, B B1, W C4 was also a valid answer. As for the official answer, it ends in seki for brekky and B/W both have 1 point of territory. Thinking more though, I realize that with this alternate sequence both B and W have captured 1 stone each whereas in the official answer only Black has captured. i.e. the official answer is one point better for Black.
Sadaharu: In the correct answer, black has 1 point but white has none. If you sacrifice a stone at C3 white will have 1 point, so that's not correct
In the correct answer, black has 1 point but white has none. If you sacrifice a stone at C3 white will have 1 point, so that's not correct