关于only 的一个问题

For the first time in the modern era, non-Hispanic Whites are officially a minority in California, which amounts to a little less than half the population of the state, down from nearly there-quarters only a decade ago.

1. which amounts to a little less than half the population of the state, down from nearly three-quarters only a decade ago  
2. which amounts to a little less than half the population of the state, down from a decade ago, when it was nearly three-quarters
3. and that amounts to a little less than half the population of the state, down from a decade ago, when they were nearly three-quarters
4. amounting to a little less than half the population of the state, down from nearly three-quarters a decade ago
5. amounting to a little less than half the population of the state, down from what it was a decade ago by nearly three-quarters

答案是 D 

我一开始是想选D的，但是注意到 原文 A 有一个 only 强调 a decade ago， 而其它四个选项都漏掉了 only，

我记得有一条语法规律，说是 only , first 等一系列强调作用的词  位置不能变，不能随意删改。

请高手确认。  我翻了一下以前的记录，没找到专门讨论only的帖子，如有，也请好心人加个连接。

谢谢