单项选择题

We all accept that killing is in general wrong, but virtually all of us also recognize certain exception—that is, concede that there can be instances in which killing is permissible. In addition to accepting the obvious permissibility of killing microbes and plants(except then this is objectionable for either instrumental or impersonal reasons)most people believe that it can be permissible in a variety of circumstances to kill animals, and also that it can be permissible to kill other human beings in self-defense and in appropriate conditions in war. There are four distinct categories into which we may sort most or all instances of killing for which there may be a reasonable justification. Perhaps the most contentious category consists of cases in which killing would simply promote the greater good—for example, a case in which killing one person would prevent the killing, or the deaths, or the deaths, of a much greater number of people. The second category consists of cases in which an individual has done something that has lowered the moral barriers to harming him, or compromised his status as inviolable, or made him liable to action that might result in his death. Cases in which killing might be thought to be justified for this sort of reason include killing in self defense, killing in war, and killing as a mode of punishment. The third category of possibly permissible killing consists of cases in which the metaphysical or moral status of the individual killed is uncertain or controversial. Among those beings whose nature arguably entails a moral status inferior to our own are animals, human embryos and fetuses, newborn infants, congenitally severely retarded human beings, human beings who have suffered severe brain damage or dementia, and human beings who have been in irreversible coma. These are beings that are in one way or another "at the margins". The fourth and final category comprises cases in which death would not be harm to an individual but instead a benefit. In many such cases, the individual for whom death would be a benefit also desires to die and may request to be killed or helped to die. The practical issues that arise under this heading are suicide, assisted suicide and euthanasia. The passage states that______.