It was about 600 years ago the first clock was made.A. thatB. untilC. before D. when

问题补充：

It was about 600 years ago the first clock was made.

√A. that

B. until

C. before

D. when

答案：

@@√A. that@@B. until@@C. before @@D. when

这是一个强调句，把it was … that这个结构去掉，我们得到的是about 600 years ago, the first clock was made。根据强调句的要求，后面只能用that来连接。