only在这句中为什么不倒装

Only____toborrowmagazinesfromthereadingroom.A.aretheteachersallowedB.theteachersareallowed答案是选B，谁能帮我回答一下...

Only____to borrow magazines from the readingroom.
A.are the teachers allowed
B.the teachers are allowed
答案是选B，谁能帮我回答一下

如果only位于句子的开头，但它们作形容词用不是修饰介词短语或状语从句，而是修饰名词或代词则句子不需倒装：Only he managed to pass the exam. 只有他设法通过了考试。

“某些副词(短语)＋助动词＋主语＋谓语”的倒装结构

这些副词有(just)then, soon, often, only(in this way, then), so(＋形容词), especially, near(＋地点名词或短语), early(＋时间状语), in vain, many a time等等

1) So little about mathematics that the lecture was completely beyond me.

〔A〕 I know
〔B〕 I knew
〔C〕 do I know
〔D〕 did I know

2) a partial answer.

〔A〕 Will be found in this chapter
〔B〕 In this chapter found will be
〔C〕 In this chapter will be found
〔D〕 This chapter will be found in

3) So abundant and so prominent the material benefits of science are 〔A〕 that in the eyes of many 〔B〕 these seem to be the only 〔C〕 benefits which she brings 〔D〕 .

这里的only是用来修饰助于-老师的，不是修饰动词，当然不用提前， 就像唐僧唱歌的：only you can陪 me取西经。。。。。。。only是用来修饰you的。唐僧英语水平很高的。。。

only修饰句子的状语（从句）位于句首时，句子（主句）要用部分倒装。如： Only then did I realize the importance of English． Only when a child grows up does he understand his parents’ intentions． 但如果only修饰的是句子的其它成分，则无需倒装。如： Only socialism can save China． (only修饰的是句子的主语，故仍用正常语序。)

the teachers are allowed to borrow magazines from the readingroom. only放在teachers前后都一样意思。 但是选a的话就变成疑问句型了。