(;AB[eb]AB[fb]AB[gb]AB[hb]AW[ib]AW[jb]AB[kb]AB[lb]AB[ic]AB[jc]AW[hc]AW[gc]AW[fc]AW[ec]AW[kc]AW[lc]AW[je]AW[mb]AW[mc]AB[db]AB[cb]AB[bb]AB[ab]AW[dc]AW[cc]AW[bc]AW[ac]LB[ia:A]LB[ja:B]AW[ma]AW[hd]AW[ie]AW[kd]C[Best move for B.]AP[goproblems] (;B[ja];W[la]C[K19 is wrong because now if B ignores this ko threat, the k19 stone will be on the wrong place. After W L19, B's stone would be better on J19. So J19 lowers the value of M19 as a ko threat.]) (;B[ia]C[Ok. If you had done k19, W's m19 ko threat is bigger.RIGHT]))