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

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@@√A. [email protected]@B. [email protected]@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。