Friends play an important role in our lives, and although we may take friendship for granted, we often don’t clearly understand how we make friends. While we get on well with a number of people, we are usually friends with only a very few — for example, the average among students is about 6 per person. In all the cases of friendly relationships, two people like one another and enjoy being together, but beyond that, the degree of intimacy between them and the reasons for their shared interest vary enormously. As we get to know people we take into account things like age, race, economic conditions, social position and intelligence. These factors are of prime importance, as it is often more difficult to get on with people when there is a clear difference in means, background, maturity, and/or capabilities.
Some friendly relationships can be kept on despite argument or heated discussion, but it is usual for close friends to have similar ideas, beliefs, attitudes, and interests. In fact, close friends may sometimes talk about "being on the same wavelength". It generally takes time to reach this point. And the more intimately involved people become, the more they rely on each other, People want to do friends favors and hate to break a promise. Equally, friends have to learn to put up with any annoying habits and to tolerate differences of opinion.
In contrast with marriage, there are no government-sanctioned friendship ceremonies to strengthen the association between two people. But the support and understanding of each other that result from shared experiences and emotions do seem to create a powerful bond, which can overcome difference in background, and thus break down barriers of age, class, or race.

According to the author,（）

A．all those who get on well with each other are friends
B．friends are closer than people who just get on well with each other
C．everyone understands clearly how to make friends
D．every student has six friends