根据下列内容，回答206-210题。
Social change is more likely to occur in societies where there is a mixture of different kinds ofpeople than in societies where people are similar in many ways. The simple reason for this is thatthere are more different ways of looking at things present in the first kind of society. There aremore ideas, more disagreements in interest, and more groups and organizations with differentbeliefs. In addition, there is usually a greater worldly interest and greater tolerance in mixedsocieties. All these factors tend to promote social change by opening more areas of life to decision.
In a society where people are quite similar in many ways, there are fewer occasions for people tosee the need or the opportunity for change because everything seems to be the same. And althoughconditions may not be satisfactory, they are at least customary and undisputed.
Within a society, social changes is also likely to occur more frequently and more readily inthe material aspects of the culture than in the non-material, for example, in technology rather thanin values; in what has been learned later in life rather than what was learned early; in the lessbasic and less emotional aspects of society than in their opposites; in the simple elements ratherthan in the complex ones; in form rather than in substance; and in elements that are acceptable tothe culture rather than in strange elements.
Furthermore, social change is easier if it is gradual. For example, it comes more readily inhuman relations on a continuous scale rather than one with sharp difference. This is one reason whychange has not come more quickly to Black Americans as compared to other American minorities,because of the sharp difference in appearance between them and their white counterparts.
The passage is mainly discussing__________.

A.certain factors that determine the ease with which social changes occur
B.certain factors that promote social change
C.the necessity of social change
D.two different societies