一道高中英语单项选择题？

it is...that 是强调句 强调句的特点是：把it is 和that去掉能还原成一个完整的句子。这个句子把it was 和that去掉可写成：The first clock with a face and an hour hand was made about 6oo years ago.因此这句话是强调句 故用that。 在高中英语中和强调句易混，但用when的句型是:it is +时间点+when从句.在这个句型中it表示时间 作主语，when（当...时）引导时间状语从句。如:以下三题 都用when
[1]It is about 9:30 pm ______ he comes back home after three evening
classes . (2005崇文二模)
A. when B. before C. that D. since
[2]It was twelve o’clock at midnight _____ they arrived at a lonely

village
A. that B. before C. since D. when (2004石家庄二摸)
[3]---Did Jack come back early last

night ?
---Yes .It was not yet eight o’clock _______ he arrived home .
A. before B. when C. that D. until (2005广东卷)
在以上三题中 不能把it is和when去掉。

恩~这个是一种特殊的句式
叫做强调句型 它的目的是强调 600年 这个成分
强调句型的特征是 你把 IT WAS 和 THAT去掉以后 还是能成为一