EVENT
BOARDSIZE 19
SETUP W D17 B17 C16 C15 B15 D14 D13 C13 B13 C3 
SETUP B C18 C17 D16 D15 E17 E14 Q4 Q16 
COM
Problem 99. Black to play.  

A joseki has been played out in the upper left corner and White
has gotten a thick position on the upper left side. However,
Black has kept sente. Where should he play now ? 
ENDCOM
B 1 D4
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Since White's stone in the lower corner is low, Black can strike
at 1. The reason for Black chose the joseki in the upper left
was because he had this move.
ENDCOM
  VAR
  B 1 Q10
  COM
  Failure.

  It is not appropriate for Black to make a sanren-sei in this
  position.
  ENDCOM
  W 2 D6
  COM
  White will extend to 2, making an ideal position on the left
  side. If Black wanted to make a sanren-sei, he should have played
  differently in the upper left corner.
  <=
  ENDCOM
  ENDVAR
W 2 C4
COM
Crawling with 2 is the right direction.
ENDCOM
B 3 D5
W 4 E2
B 5 G4
COM
After Black 5, White's thickness has been neutralised.
<=
ENDCOM