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.

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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.

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