comiluv: B S2 is the only necessary move because (no ko) is not stated in the problem
caranthir: "No ko" means that the killing must be unconditional and not depend on winning a ko using some imaginary non-local ko threat(s). Therefore if S2, T2, the attacker must then continue with S1 in order to prevent ko.
B S2 is the only necessary move because (no ko) is not stated in the problem