请帮忙看下这个句子的主谓宾. 我怎么找不到谓语

29. Crucial in
The debate over bilingual education centers on the issue of whether the
United States should foster the idea of single common language, an idea,
some believe, that has in the past been crucial in binding diverse
constituencies together.
(A) been crucial in binding diverse constituencies together
(B) been crucial as a binding together of diverse constituencies
(C) been crucial to bind together constituencies that are diverse
(D) become crucial in binding together diverse constituencies
(E) become crucial to bind together constituencies that are diverse
A is correct answer.

我纠结的不是答案考点. 而是找不到句子结构, 主语是debate吧. 谓语是哪个? has前面的that指代谁. 请进来的CDER帮忙分析一下, 谢谢!!

刚查了,同意3L说法

(A) been crucial in binding diverse constituencies together
(B) been crucial as a binding together of diverse constituencies
(C) been crucial to bind together constituencies that are diverse
(D) become crucial in binding together diverse constituencies
(E) become crucial to bind together constituencies that are diverse
我试着解释一下 不知道对不对 首先通过比较发现没有明确的语法考点 那么通过比较进行逐项排除；D E比较发现主要区别一个在于不定时和介词短语 一个在于后面的从句和形容词； 不定时用于修饰动词 主语为句子主语表示目的和将来时（pre笔记） 不合逻辑！因为不能是idea to bind 同时adj优于从句 排除E 同理排除C； 也就是根据动作发出者和简洁性排除C E； 剩下A B D； 根据原意 idea应该是crucial in constituencies 而B中的意思是ideas as a binding也就是binding成为了名词 不合逻辑 同时严重改变原文 排除B； 也就是通过逻辑关系和尊重原文重心的角度排除B； 之后A D； 通过比较发现一个是been crucial 一个是become crucial； 由于是adj 所以构成系表结构 优选been 而且idea become crucial 不合逻辑和become的一般用法 另外together按照句尾更加符合一般的用法 所以优选A

所以考点在于简洁性 逻辑

拙见 NN指点！