(;
GM[1]
SZ[19]
AW[fq][dr][cr][cq][cp][cn][jq][jp][lp][ln][jn][jl][kl][mg][lf][mi][lk][mk][po][qp][qm][ql][qk]
AB[dd][dp][do][dq][eq][er][fp][ip][iq][io][lm][ll][kk][kj][kh][pd][og][pj][pk][pl][nl][on][oo][oq]
TR[kk][kj][kh][lf][mg][mi][mk][lk]
C[Problem 20. Black to play.  

Both the black and white marked stones are floating in the center. It is Black's move. What should he do ? ]
(;B[jf]
TE[2]
C[Correct Answer.  

Black should attack with the knight's move of 1. ]
;W[ld]
C[After the exchange of 2 ... ]
;B[jd]
C[... for Black 3, ... ]
;W[nd]
C[... White makes shape with 4. but ... ]
;B[pc]
C[... Black secures the corner and a thick position with 5. White is in trouble. <= ]
)
(;B[ld]
C[Failure.  

Black 1 is in the wrong direction. ]
;W[jf]
C[It induces White to save her stones, while leaving Black's stones seriously injured. <= ]
)
)