题目内容：
根据下面资料，回答题 Drawing a picture is the simplest way of putting an idea down on paper. That is（21）men first began to write six thousand years ago or（22 ）.
The alphabet we now use（23）down to us over a long period of time. It was developed from the picture-writing of ancient Egypt.
Picture-writing was useful in many（24）.It could be used to express ideas as well as（25）.
For example, a drawing of a（26）meant the object“man”. （27）a drawing of a man lying on the ground with a spear in him meant“（28）”
Besides the Egyptians, the Chinese（29）the American Indians also developed ways（30）writing in pictures.
But only（31）much could be said this way. Thousands of pictures would have been needed（32）express all the ideas that people might have.
It would have taken many thousand more to express all the objects（33）to men. No one could（34）so many pictures in a lifetime. Nor could anyone learn the meaning of all（35）drawings in a lifetime.
第（21）题选 A．when
B．because
C．where
D．how
参考答案：
答案解析：