(;
GM[1]
SZ[19]
AW[jd][nd]
AB[dd][hc][pd][pf]
C[Problem 3. Black to play.  

White has made a three-space extension along the fourth line. How should Black invade ? ]
(;B[ld]
TE[2]
C[Correct Answer.  

Invading on the fourth line with 1 is an effective move. It will now be hard for White to make sabaki when she attaches on the third line. <= ]
)
(;B[lc]
C[Variation.  

Black plays on the third line with 1 because he wants to link up with his allies. ]
;W[ld]
C[However, it will be easier for White to make sabaki when she attaches with 2. <= ]
)
)