这个定语从句为什么要用“which”?New zeland has a population of

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因为此句是非限定性定语从句,
that与which的用法区别中有提到：1.引导非限制性定语从句时,通常要用which
例句：The current,which is very rapid,makes the river dangerous.水流湍急,使这条河很危险.
He has to work on Sundays,which he doesn’t like.他得在星期天工作,他是不喜欢这样的.
The London team,which played so well last season,has done badly this season.伦敦队上一个季度打得很好,这个季度却打得很差.
2.直接放在介词后作宾语时,通常要用which：
She may be late,in which case we ought to wait for her.她可能晚到,那样我们就要等等她.
The documents for which they were searching have been recovered.他们找寻的文件已找到了.
This morning some port wine came,for which I have to thank you.今天早上送来一些波尔图葡萄酒,为此我得向你道谢.
注：有时“介词+which”引导的定语从句可以转换成“介词+which+不定式”结构：
He had only the long nights in which he could study.=He had only the long nights in which to study.他只有漫漫长夜可用来学习.
而此句中,有逗号隔开,是非限定性定语从句的标志,且关系词which前有介词of,故用which.
希望那个你能明白!
======以下答案可供参考======
供参考答案1:
非限定性定语从句,所以用which
供参考答案2:
是 ,of which,这是分隔式定语从句，population的介词是of，which指代提到过的population，没有用that的。
供参考答案3:
关键是of后面，决对不能放that