——map dated back to about 3000 B.C

题目

——map dated back to about 3000 B.C
A,The oldest known.B,It was the oldest known.
为什么选A

答案

选A.
The oldest known map dated back to about 3000 B.C .
句意：已知最古老的地图回溯到公元前3000年.
dated back to 无被动形式,所以不可能是过去分词形式作定语,只能是过去式形式的谓语动词.
如果选B,谓语动词是was,那么后面的dated back to就是非谓语,这是说不通的.