(;
GM[1]
SZ[19]
AW[ic][nd][nc]
AB[dd][gc][oc][pd][pf]
C[Problem 21. Black to play.  

Even when White has the two stones on the right, the four-space extension is a bit wide. Where does Black invade ? ]
(;B[ld]
TE[2]
C[Correct Answer.  

Black 1 shows good go sense. ]
;W[lb]
C[If White tries to link up her stones with 2, ... ]
;B[kc];W[kb];B[jc];W[jb];B[id]
C[... Black presses her in a low position with the moves to 7. <= ]
)
(;B[lc]
TE[2]
C[Correct Answer.  

Black could also invade deeply with 1. ]
;W[kd];B[me];W[md]
LB[kd:2]
C[However, White can confine Black to the side with 2 and 4. ]
;B[ld];W[le]
C[This will lead to an all-out fight. <= ]
)
)