(;
GM[1]
SZ[19]
AW[db][fc][ic][qj]
AB[cc][dd][df][cj][mc][pd]
C[Problem 68. White to play.  

Black has stones on both the left and the right. Where should White invade ? ]
;W[qc]
TE[2]
C[Correct Answer.  

White 1 is good timing. ]
(;B[qd]
TR[qj]
C[If Black 2, his thickness facing the bottom is negated by White's marked stone. ]
;W[pc]
LB[od:C][oc:B][rc:A]
C[Next, Black can play A, B, or C. <= ]
)
(;B[pc]
C[Variation.  

There are times when Black might want to block with 2, but ... ]
;W[qd]
C[... the black stones are left with a heavy shape. <= ]
)
)