It was about 600 years ago ________ the first clock with a face

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
【正确答案】：A
【题目解析】：该句为强调句，强调句的特点是去掉It was ……that……以后句子结构完成。即，the first clock with a face and an hour hand was made about 600 years ago 句子结构完整。