EVENT
BOARDSIZE 19
SETUP W D16 F17 D10 D4 L3 L5 L8 
SETUP B K16 Q16 Q14 R9 N7 O4 Q3 
MARK \t@L8 A@F4 B@F3 C@E2
COM
Problem 123. Black to play.  

White has just played the marked stone, mapping out a huge moyo
on the left, so the lower left is now the most important place
to play. But where should Black play: at A, B, or C ? 
ENDCOM
B 1 F4
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black should approach high with 1.
ENDCOM
  VAR
  B 1 F3
  COM
  Failure.

  White answers Black 1 ...
  ENDCOM
  W 2 F5
  COM
  ... by capping at 2.
  ENDCOM
  B 3 C3
  COM
  If Black invades the corner with 3, ...
  ENDCOM
  W 4 D3
  B 5 C4
  W 6 C5
  B 7 B5
  W 8 C6
  B 9 B6
  W 10 C7
  B 11 D2
  W 12 E2
  B 13 C2
  W 14 E3
  MARK \t@L8
  COM
  ... after the sequence to 14, the marked stone makes a huge moyo
  with White's other stones on the left.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 E2
  COM
  Failure.

  If Black 1 here, ...
  ENDCOM
  W 2 F3
  COM
  ... White makes a bigger moyo with 1.
  <=
  ENDCOM
  ENDVAR
W 2 D6
COM
If White 2, ...
ENDCOM
B 3 J4
MARK A@L7
COM
... Black extends to 3, aiming to exploit the thinness of White's
two-space jump by attaching at A.
ENDCOM
W 4 K6
COM
White will defend at 4, but ...
ENDCOM
B 5 H6
COM
... Black plays 5, wiping out much of White's moyo.
<=
ENDCOM