(;
GM[1]
SZ[19]
AW[ic][nc]
AB[dd][gc][pf][pd][qi]
C[Problem 20. Black to play.  

White has made a four-space extension along the top. Where should Black invade ? ]
(;B[kc]
TE[2]
C[Correct Answer.  

Black 1 is the vital point to invade. In this case, it is even better than A. ]
;W[ie]
C[If White jumps to 2, ... ]
;B[ke]
C[... Black follows with 3. ]
;W[me];B[kg]
C[After 5, Black can attack on the left or the right. <= ]
)
(;B[lc]
LB[kc:@]
C[Failure.  

@ is even better. <= ]
)
)