(;
GM[1]
SZ[19]
AW[bb][cb][db][dc][ed][fc][dh][cl][cr][dq][eq][gq][jq][qj]
AB[cc][bc][ad][be][ce][de][dd][df][ec][bq][cq][dp][co][fo][oq][qp][pd]
LB[nc:A][qf:B][qm:C]
C[Supplement to Problem 120: White's best move.  

Instead of A and C in the Problem Diagram, ... ]
;W[bg]
TE[2]
C[... White should have played 1. This move threatens the eye shape of the black stones above, so if Black invades on the left side, the fighting could spill over the top and the black stones there could get into trouble. For that reason, White 1 is a good move because it reinforces her thin stones on the left. <= ]
)
)