听力原文： 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 some 4000 years ago, because they could write and leave written records fro 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 write. Sometimes, of course, even if the people cannot write, they may know something of the past. For most people are proud to tell what their fathers did in the past. This we may call 'remembered history'. Some of it has now been written down. It is not so exact or so valuable to us as written history, because words are much more easily changed when used again and again in speech than when copied in writing. But where there are no written records, such spoken stories are often very helpful. Why do we know very little about the central Africa 200 years ago?