帮忙分析下句子结构和that用法 麻烦了 我很急 速且优者得

题目

帮忙分析下句子结构和that用法 麻烦了 我很急 速且优者得
Computer and video systems,for example,could be used by professional dog trainers to determine the mood of dogs that they were required to approach.(07福建卷E篇)
74.The underlined word "they" refers to_____
A.the dogs B.the trainers C.the systems D.the researchers
choose B
THAT 引导什么从句?

答案

..could be used by professional dog trainers （to determine the mood of dogs）（that they were required to approach）括号里的内容可省,两个括号地位是一样的,所以括号里的是修饰前面的主语trainers.