1.it is about 600 years ago __ the frist clock was made.(that until when since)

问题描述:

1.it is about 600 years ago __ the frist clock was made.(that until when since)
2.it was eight'clock ___ he came to school.
it was at eight ___ he came to school.(that before since when )

that , when , that
第一个跟第三个都是强调句,it is/was ...that
第二个是时间状语从句