OG16 SC51老题新问，看过论坛关于本题几乎所有讨论，仍有疑问不能解决，急盼指点回复！

Neuroscientists, having amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood, are now drawing solid conclusions about how the human brain grows and how babies acquire language.

A. Neuroscientists, having amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood, are

B. Neuroscientists, having amassed a wealth of knowledge about the brain and its development from birth to adulthood over the past twenty years, and are

C. Neuroscientists amassing a wealth of knowledge about the brain and its development from birth to adulthood over the past twenty years, and are

D. Neuroscientists have amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood,

E. Neuroscientists have amassed, over the past twenty years, a wealth of knowledge about the brain and its development from birth to adulthood,

本题答案是A。我看了很多之前的讨论帖，关于D、E的错误大家都给出了很充分的解释。尤其是Ron关于comma+V-ing的用法有几个前提条件的解释非常清楚。为方便讨论，这里再贴一下Ron的解释：this sort of modifier should actually satisfy TWO requirements:
1) it should apply most nearly to the subject of the preceding clause (as you've said); and, even more importantly,
2) it should have one of the following RELATIONSHIPS to that clause:
* immediate consequence（when we say "immediate consequence, we mean a consequence that is proximate, immediate, and produced as an essentially unavoidable result of the main action.）
* simultaneous, but lower-priority, action

但是关于A选项本身，我还有些疑问。有人说这里的having amassed是定语，有人说是状语，不管是状语还是定语，急切的请教一下大家，是不是v-ing放在了主句动词之前，就不再有Ron提到的那些前提条件的要求了？大家说D、E错误是因为have amassed与drawing不是同时发生，且没有直接因果关系，那么当-ing转去主句动词之前，having amassed与 are now drawing一样没有这些关系，可A就正确了，所以是不是这种情况就不再要求两动作同时发生且有直接因果关系了呢？

还是说having done作为非限定性修饰，是一个v-ing的特例，除了这种用法，其他情况的v-ing无论定语还是状语，无论位置在哪，都还是要遵循Ron提到的法则。

期待大家的讨论和指教！！！

这里再补充一下，之前讨论里有很多人提到D、E改变了句子的重心因此直接排除，我觉得这个方法是不可靠的，有运气的成分。想要表达什么重点完全取决于说话人的意图，很难一开始就判断哪部分是重点。manhattan第六版里有一个正确用法的例句就是个很好的的例子。

Scientists have found high levels of iridium in certain geological formations around the world, suggeting the  cataclysmic impact of a meteor millions of years ago.