[单项选择]

If we were asked exactly what we were doing a year ago, we should probably have to say that we could not remember. But if we had kept a book and had written in it an account of what we did each day, we should be able to give an answer to the question. It is the same in history. Many things have been forgotten because we do not have any written account of them. Sometimes men did keep a record of the most important happenings in their country, but often it was destroyed by fire or in a war. Sometimes there was never any written record at all because the people of that time and place did not know how to write. For example, we know a good deal about the people who lived in China 4 000 years ago, because they could write and leave written records for those who lived after them. But we know almost nothing about the people who lived even 200 years ago in central Africa, because they had not learned to wrtie.
Sometimes, of course, even if the people cannot write, they may know so
A. history based on a person's imagination
B. stories of important happenings passed down from mouth to mouth
C. songs and dances about the most important events
D. both B and C