单选题 About the sixth century Ａ．Ｄ．when few Europeans could read, the Chinese ______ Paper.
   A．invented    B．had invented
   C．have invented D．had been invented
【正确答案】 B
【答案解析】在公元6世纪之前发生的事情，是过去的过去，所以用过去完成时。