It's definitely A. In my opinion, there's a clear dividing line between the life of a plant and the life of a person. However, the line gets fuzzier(again, in my opinion) the more developed the life form gets. By developed, I mean able to feel and react to pain, and generally close to sentience. Also, to take the comparison a step further, would you kill a street junkie with AIDS if it meant saving the life of another human? If so, would you do it under any circumstance? There's a BIG difference between killing someone who is threatening a third person's life and cold-bloodedly slaughtering him to indirectly save somebody's life.