这个英语题为什么选A不选D

题目

这个英语题为什么选A不选D
It was about 600 years ago ( )the first clock with a face and an hour hand was made.
A that
B until
C before
D when

答案

看从句：the first clock with a face and an hour hand was made.是一个不完整的被动句.所以要用that（关系代词）来补充完整.when是关系副词,如果从句时一个完整的句子（主谓宾/主系表/……皆全）才用 改it + be动...