EVENT
BOARDSIZE 19
SETUP W C6 C17 C18 D16 D15 E15 E14 E13 C14 B14 B13 C12 F16 R17 R15 R12 S6 Q5 Q3 K3 
SETUP B C4 E3 B18 B17 C16 C15 B15 D14 C13 D17 E17 E16 F15 G16 M17 P17 R10 R7 S7 F14 
MARK \t@C6
COM
Problem 91. Black to play.  

White has just made a wide extension from her thickness above
with the marked stone. Where should Black play next ? 
ENDCOM
B 1 F13
MARK \t@C6 A@E12
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is a strong move because it expands Black's moyo at the
top and threatens to force at A. Instead of playing the marked
stone, White should have turned at 1.
<=
ENDCOM
  VAR
  B 1 H3
  COM
  Failure.

  Responding to the marked stone by extending to 1 is not good.
  ENDCOM
  W 2 F13
  MARK \t@C6 A@C7
  COM
  White turns at 2. If White played the marked stone at 2, Black
  would extend to A, neutralising White's thickness above.
  <=
  ENDCOM
  ENDVAR