(3) 现在分词(having done)在句尾用逗号隔开，必须要注意的一点是这个结构从来都没有失去过“做定语”的功能，只是优先做状语而已，即优先修饰到前面主句的主语和谓语。

　　例：OG12-21 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,

　　OG对D项(代入原句：Neuroscientists have amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood, now drawing solid conclusions about how the human brain grows and how babies acquire language.)的解释: The final descriptor in present tense,now drawing conclusions....does not fit the opening clause, which is in present-prefect tense(have amassed a wealth)and seems to modify adulthood. OG在这个解释里说明了drawing是可能修饰到adulthood的。Key: A

　　关于which的指代：

　　which的指代其实并没有固定死规则，不一定只指代最近的名词，跟所有代词一样，有就近指代的的倾向，但是具体修饰对象，应该根据各种线索或者逻辑含义来判断：(以下收录官方材料中能对于which的使用)

　　(1) 尽管A of B的结构，which没有就近修饰。

　　561. One pervasive theory explains the introduction of breakfast cereals in the early 1900s as a result of the growing number of automobiles, which led to a decline in horse ownership and a subsequent grain glut; by persuading people to eat what had previously been horse feed, market equilibrium was restored.

　　(2) which不一定跳过插入语指代：

　　462. Judge Lois Forer’s study asks why some litigants have a preferred status over others in the use of a public resource, the courts, which in theory are available to all but in fact are unequally distributed between rich and poor.

　　(3) 通过单复数判断which的所指：

　　173. (GWD-10-Q18) In analyzing campaign expenditures, the media have focused on the high costs and low ethics of campaign finance, but they have generally overlooked the cost of actually administering elections, which includes facilities, transport, printing, staffing, and technology.

　　205. (GWD-9-Q17) Unlike most other mergers in the utility industry, which have been driven by the need to save money and extend companies’ service areas, the merger of the nation’s leading gas company and leading electric company is intended to create a huge network for marketing the utilities in question as states open their utility markets to competition.

　　(4) 通过时态区别which的所指：

　　488. Like the government that came before it, which set new records for growth, the new government has made laissez-faire capitalism its cornerstone.

　　关于比较—主语比较：

　　对只有主谓(如果主语有定语成分，必须要that/those来指代)，没有宾语的句子，因为没有宾语，只可能有主语比较，因而不需要重复do来确认这是一个可发出动作的主语，所以其比较形式如下：

　　I eat faster than you. VS I eat apple faster than you do. 如果写成 I eat apple faster than you 按GMAC的理解，从语法上就变成我吃苹果比吃你快，成了宾语比较。逻辑上显然不通。

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(编辑：马荟)