听力原文： In the Middle Ages two was the official number of meals because of the clergymen's insistence on fasting. But this official pattern was frequently changed for some practical reasons. For example, for health reasons, laborers, the sick, the very young and the every old were permitted a third meal in the morning, hence the breaking of the fast or breakfast. By the seventh century three meals a day had become the norm at all levels of society. It remains the publicly adopted norm even today, and there may be two reasons for this general acceptance. According to the result of the computerized tests of the body's energy needs, physiologists conclude that the three-meal pattern is suitable nutritionally to the average adult. The second reason is related to the human association. The number 3 has had a mystical, comforting significance for Western minds and consequently we speak with great respect of the Trinity and we take the triangle as geometrically the perfect figure. In fact, considering the prevalence of snacking in America, it would be more correct to say that the norm is six or seven "meals" a day.
When did three meals a day become a widely-accepted norm at all levels of society？

A. By the seventeenth century
By the seventh century
C. By the end of last century
D. In the Middle Ages