the opening ceremony is a great occasion.it is essential ___

the opening ceremony is a great occasion.it is essential _______for that.
A for us to be prepared B that we are prepared.
..可是我觉得应该是 for us to prepare
答案说如果是B的话 应该原句是 it is essential that we should prepare for that opening ceremony （那如果按照是A的逻辑 是被动的话 这里不应该就是...should be prepare）
为什么不是 for us to prepare
it is essential ___（for us to be prepare）____for that.
这里的it 指us的话。为什么要用被动。

it is essential ___（for us to be prepared）____for that.里的it 是形式主语代替真正的主语to be prepared for that放在句首,而不是指us,那个for us 是essential的补语,不影响动词的形式.
另外,to be prepared的be prepared不是被动语态,而是动词的过去式作形容词表示“准备好的”.
类似的句子还有：
We are prepared for the final esamination.(对于期末考试,我们做了充分的准备.)
这个里面所涉及的语法点有：it做形式主语,不定式短语,动词的过去式的意义
不知道这么解释,LZ清楚了没,欢迎交流~