About the sixth century A. D. when few Europeans could read, the Chinese ____paper. (2 分) A. invented B. had invented C. have invented D. had been invented.
问题描述：
About the sixth century A. D. when few Europeans could read, the Chinese ____paper. (2 分)
A. invented
B. had invented
C. have invented
D. had been invented.
答
B这是一个过去完成时,发生在 前半句话( 它已发生在过去时了）的前面