It was ordered that all the soldiers ______ to the front.
A. should send B. must be sent C. should be sent D. must go
What I am wondering is why not choose B ?The key of it is C.

It should be "C" if we consider the correct use of grammar.

It is something about the use of Subjunctive Mood(虚拟语气).

When some verbs are followed by a Object clause(宾语从句), or are used in a Subject Clause(主语从句), Predicative Clause(表语从句) or Appositive Clause(同位语从句),the verb in the clause must be used in its Subjunctive Mood. The structure is like this：

should do / should be done

should not do / should not be done

And the verb "order" belongs to this group of verbs. Other verbs that must be used in the same way include "command/ suggest,advise, insist / require ,request"

This group of vers are just a few. You just need to remember the Chinese meaning they have :命令，建议，要求.

Take the sentence above for exapmle:

It was ordered that all the soldiers ______ to the front.


这是一个有it当形式主语引出一个主语从句，起真正主语在后头

that all the soldiers ______ to the front.
因主语较长，放在句前显得头重脚轻不协调，不符和英语表达的习惯，所有后置了。