--When the abacus ? -- In the sixth century by the Chinese people.
A. is, invented
B. did, invent
C. was, invented
D. does, invent
网友回答
A. is, invented
B. did, invent
(答案→)C. was, invented
D. does, invent
解析：
此题考查的是被动语态，由be+过去分词。算盘是人发明的，它是动作的承受者。发明在过去，故用was invented,选C。