OG12-SC-21 C选项的解释！！哪位大神再帮我解释下！！

OG12-SC-21
21 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,
答案选A我是懂的，其他几个选项为什么错我也懂，除了C。
其实要是不看OG的解释C我也排除了，但是看了解释我就不懂了···
解释是这样的：Amassing, like having amassed, functions as an adjective, not a verb; the sentence therefore lacks the first main verb implied by the compound verb construction and are now drawing...
首先，我觉得having amassed不是做形容词，而是分词做伴随，解释说明neuroscientists的，是说neuroscientists在过去的20年已经做了····
同样，amassing我觉得也是分词修饰neuroscientists， 现在分词是表示主动的现在的一种持续性动作，而后面时间状语是over the twenty years, 应该用完成时，所以amassing不对。而且amassing直接修饰主语，中间没有逗号隔开，此分句不是一个完整的句子，于后面and are···无法形成并列。
我主要不明白的就是amassing在这里为什么是形容词。哪位sama能帮我解释一下啊~~~万分感谢！！！