(;
GM[1]
SZ[19]
AW[ec][gd][jc][qj][qm][cp][dq][fp][ck]
AB[cc][de][od][qd][qh][po][pq][kp][co]
C[Problem 105. Black to play.  

There are a number of big extensions Black can play. Which one is the biggest ?? ]
(;B[ch]
LB[dl:B][cg:A]

TE[2]
C[Correct Answer.  

If White makes a checking extension at A, the two black stones in the upper left will come under a strong attack. Therefore, Black extends to 1 to prevent this. The reason Black extends so tightly is because he intends to make a shoulder hit at B next. ]
;W[mc]
C[White 2 and ... ]
;B[rn]
C[... Black 3 are the natural continuation. <= ]
)
(;B[lc]
C[Failure.  

The checking extension of Black 1 is also a good move, ... ]
;W[cg]
C[... but White 2 is too big to allow; the Black stones there could find themselves uprooted and they would have to run for their lives. <= ]
)
(;B[ci]
C[Failure.  

Black 1 here ... ]
;W[ek]
LB[cg:A]
TR[co]
C[... could provoke White to jump to 2 and the marked black stone would be swallowed up in the white moyo. After this, White could aim at the invasion at A. <= ]
)
)