(;
GM[1]
SZ[19]
AW[ic][md][nc][od]
AB[dd][gc][oc][pc][qd][qf]
LB[ie:A]
C[Problem 27. Black to play.  

Black attacks with the marked stone, aiming to invade. If White doesn't defend by playing A, where does Black invade ? ]
(;B[lc]
LB[nb:A]

TE[2]
C[Correct Answer.  

Black 1 is the vital point. Black threatens to link up with his stones on the right by playing at A. ]
;W[mb]
C[White must defend at 2. <= ]
)
(;B[kc]
C[Failure.  

An invasion at Black 1 is also possible, but ... ]
;W[jd];B[kd];W[ke];B[ib];W[hb];B[jb];W[hc];B[gb];W[le];B[ha];W[he]
C[... with the moves to 12, White gets a thick shape in the center of the board. <= ]
)
)