定语从句疑问和剖析

The first trip which was expected to take place in the middle of this year was pushed back to at least 2021

because of the coronavirus pandemic and the new safety measures it requires.

这是CNN新闻播报员说的。

1) 句子开头处trip后的which是不是应该改成that？由于先行词有first限定。对吗？

2）句末处，定语从句the new safety measures it requires. - 假如it是定语从句的主语，那样it指代什么呢？ 假如it不是主语，是不是应该去掉it,改成关系代词which或that作主语，同时require应该是不及物动词，

那样到底如何样理解剖析呢？

先行词被序数词、最高级形容词与不定代词修饰，关系代词一般用that，但这并不绝对。所以用which不算错，用that更为容易见到。

it requires为定语从句，省略了作宾语的关系代词。it指代 the coronavirus pandemic。