raytsh: Omitting the "no ko" suggests, as it does with hundres of other Tsumego here, that ko is a valid (if not the only) solution. It is odd that this then is marked as "incorrect".
caranthir: I must disagree. Telling if there's a ko or kos required, or if a straight kill exist, is just a hint to the student, a modifier of the difficulty of the problem so to speak. Omitting the hint does not imply that any clearly inferior (compared to the optimal) approach be considered valid.
Omitting the "no ko" suggests, as it does with hundres of other Tsumego here, that ko is a valid (if not the only) solution. It is odd that this then is marked as "incorrect".