题干
——When _____ the abacus _____? ——In the sixth century by the Chinese people.
A：is, invented
B：did, invent
C：was, invented
D：does, invent
答案(点此获取答案解析）
C
——When _____ the abacus _____? ——In the sixth century by the Chinese people.
A：is, invented
B：did, invent
C：was, invented
D：does, invent
