(;AB[ra]AB[rc]AB[rd]AB[qf]AB[qg]AB[qh]AW[ri]AW[qi]AW[pi]AW[ph]AW[pg]AW[pf]AW[rk]AW[pe]AW[pd]AW[pc]AW[pa]AW[qa]AW[nb]AW[nc]AW[sb]AB[rb]AW[sf]AB[rh]LB[rb:1]LB[sf:2]LB[rh:3]C[Black defended incorrectly in problem 11971. What can white get? (S)eki, (K)o, or an (U)nconditional kill?FORCE]LB[nf:S]LB[nh:K]LB[nj:U]AP[goproblems]
(;W[nh];B[ng]C[Prove it.]
(;W[sh];B[sg]
(;W[re];B[rf]
(;W[rg];B[ni]LB[nj:K]C[Respond to the (K)o threat?]
(;W[nj];B[sg];W[se];B[qe];W[rg]C[Excellent.RIGHT])
(;W[qe];B[nj]C[White still had an internal threat.]))
(;W[se];B[qe]C[You just wasted a threat.]))
(;W[rg];B[rf];W[se];B[sg]C[Not the best ko.]))
(;W[qe];B[re]
(;W[rf];B[sg];W[si];B[se])
(;W[sh];B[sg];W[rg];B[rf]
(;W[sg];B[se];W[si];B[sc])
(;W[se];B[sd])
(;W[si];B[se];W[sg];B[sc]))
(;W[sc];B[rf];W[sh];B[sg];W[rg];B[se]))
(;W[re];B[qe]
(;W[sh];B[rf];W[se];B[sg]C[Not the best ko.])
(;W[qd];B[rf];W[sh];B[se];W[sg];B[sc])))
(;W[nf]C[No, white can do better than seki.])
(;W[nj]C[No, white cannot kill unconditionally.]))