In following problems you need to identify last remaining place, where points could earn, or reduced. The gained points are marked in the correct answer. The lost reduction opportunities are marked in wrong variations.
An example game continuation is shown. White 1 cannot capture the black stone. Black 2 could connect black stone, but here it does not do it. White 3 may capture black stone, as stones 1 and 2 changed the whole-board position.
A string of stones is captured by playing into string's last liberty. In the diagram white stones (A) are captured when black plays into B. A stone string consists all the stones connected by lines. A string does not connect diagonally.
White may not capture black immediately after black capture, as this would produce the same whole board position as before black move. So white must play somewhere else. After this white may capture from A.