EVENT
BOARDSIZE 19
SETUP W D16 D14 D4 R5 Q5 P7 Q8 R8 O17 
SETUP B C9 Q3 R4 N3 R7 Q10 Q16 F17 K16 
MARK A@M17 B@P17 C@Q14
COM
Problem 121. Black to play.  

How should Black attack the lone white stone in the upper right:
at A, B, or C ? 
ENDCOM
B 1 P17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is a basic attacking move in this kind of position.
ENDCOM
  VAR
  B 1 Q14
  COM
  Failure.

  If Black plays 1 here, ...
  ENDCOM
  W 2 Q18
  COM
  ... White will slide to 2 and secure her stone.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 M17
  MARK A@R17
  COM
  Failure.

  Against the pincer of Black 1, White will not invade at A.
  ENDCOM
  W 2 Q14
  COM
  Instead, she will play 2 ...
  ENDCOM
  B 3 O16
  W 4 Q12
  COM
  ... and 4, attacking the black stone in the middle of the right
  side.
  <=
  ENDCOM
  ENDVAR
W 2 O16
COM
When White stands to 2, ...
ENDCOM
B 3 Q14
COM
... Black jumps to 3. White's stones are now heavy and no matter
how she defends, they will come under attack.
<=
ENDCOM