Please find the best result for Black!
(japanese rules)
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[4]KM[0.00]
PW[Weiß]PB[Schwarz]AW[cb][cc][cd][dd]AB[bb][bc][ad][bd]C[Black to play - what's the best he can do?]
(;B[ba]CR[ba]
;W[ca]CR[ca]
;B[db]CR[db]C[Black wins the capturing race RIGHT])
(;B[ca]CR[ca]
;W[ba]CR[ba]
;B[aa]CR[aa]
;W[db]CR[db]
;B[ba]CR[ba]
;W[ab]CR[ab]
;B[ac]CR[ac]C[Seki, Black wins by two points (the two prisoners). This isn't the best result!]))