【单选题】

Anthropologists commonly distinguish three forms of marriage: monogamy, the marriage of one man to one woman, polygyny, the marriage of one man to two or more women, and polyandry, the marriage of one woman to two or more men. Polygyny and polyandry are often linked under the single term "polygamy" , a marriage of one individual to two or more spouses.

Though there are many societies which permit, or even encourage, polygamous marriages, it does not follow, in such societies,that every married individual, or even that a majority of them, has more than one spouse, Quite the contrary is true, for in most, if not all, of so called polygamous societies monogamy is statistically the prevailing form. The reason for this is clear: the proportion of male to female births in any human society is roughly the same, and if this proportion is maintained among the sexually mature, a preponderance of plural marriages means that a considerable number of either men or women must remain unmarrieD、No society can maintain itself under such conditions; the emotional stresses would be too great to be surviveD、Accordingly, even where the cultural ideals do not prohibit plural marriages, these may occur on any notable scale only societies where for one reason or another, one sex markedly outnumbers the other. In short, monogamy not only prevails in most of the world’s societies, either as the only approved form of marriage or as the only feasible form, but it may also prevail within a polygamous society where, very often, only a minority of the population can actually secure more than one spouse.
In a polygynous household the husband must supply a house and garden for each of his wives. The wives live with him in turn, cooking and serving fur him during the period of his visit. The first wife takes precedence over the others. Polyandry is much rarer than polygyny. It is often the result of a disproportion in the ratio of men to women.
In sum, polygamy is not, as so frequently indicated, universally a result of human immorality. It is simply not true, in this aspect of euhure as in many others, that people who follow patterns of culture deemed immoral in our society are thereby lacking in morality. Our ideal and compulsory pattern of marriage, which holds that monogamy is the only appropriate form of marriage, is not shared by all peoples, even by some of those who regularly practice monogamy. In a great many societies, monogamy is only one possible form of marriage, with polygyny or polyandry as perfectly possible, though less frequent, alternatives.
The author believes that plural marriages occur on substantial scales only in societies in which______.
A、polygyny is not prohibited
B．one sex greatly outnumbers the other
C．most people are undereducated
D．a minority of people are very rich and powerful