EVENT
BOARDSIZE 19
SETUP W K16 O16 
SETUP B D16 H17 Q16 Q14 
COM
Problem 3. Black to play.  

White has made a three-space extension along the fourth line.
How should Black invade ? 
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B 1 M16
USER
CORRECT ANSWER
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COM
Correct Answer.

Invading on the fourth line with 1 is an effective move. It will
now be hard for White to make sabaki when she attaches on the
third line.
<=
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  VAR
  B 1 M17
  COM
  Variation.

  Black plays on the third line with 1 because he wants to link
  up with his allies.
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  W 2 M16
  COM
  However, it will be easier for White to make sabaki when she
  attaches with 2.
  <=
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  ENDVAR