I'd elaborate on the source, but I can't read Chinese.
I actually got this one wrong when I did it. Comment if there's more to say, or if you disagree with my reasons, because I'm not too certain about it (I'm sure about the answer though).
(;AW[eq]AW[dq]AW[cp]AW[cn]AB[ep]AB[fp]AB[fq]AW[qp]AW[op]LB[jq:A]LB[kq:B]C[A or B?]AP[goproblems]
(;B[el]C[[This is just here so that the board view is correct.]])
(;B[jq]C[Yup. Black can't afford to extend further, and this leaves the option of continuing at N3.RIGHT])
(;B[kq]C[Too wide; also, no graceful approach to the white corner.]))