It was in the mid-1980s that cartoons began to take off in China there were only pictured-

问题补充：

It was in the mid-1980s that cartoons began to take off in China,there were only pictured-story books.

A. when

B. after that

C. before which

D. where

答案：

A. when

B. after that

(答案→)C. before which

D. where

解析：考查非限制性定语从句，先行词为the mid1980s，故选C项。