Which of the following statements best describes the Doctrine of the Mean?
选项：

A:The Mean suggests the fundamental moral idea of moderation, balance and suitableness, and is the highest standard of moral excellence and universal harmony.;
B:The Mean is a state very difficult to reach, and therefore it begins only with the nobilities.;
C:The five social relations have nothing to do with the Mean.;
D:The Doctrine of the Mean was first advocated by Confucius.