Sixty-five million years ago, according to some scientists, an asteroid bigger than Mount Everest slammed into North America, which, causing plant and animal extinctions, marks the end of the geologic era known as the Cretaceous Period.
(A) which, causing plant and animal extinctions, marks
(B) which caused the plant and animal extinctions marking
(C) and causing plant and animal extinctions that mark
(D) an event that caused plant and animal extinctions, and it marks
(E) an event that caused the plant and animal extinctions that mark
正确答案是E，可是我不明白的是，mark为什么用谓语动词的复数形式呢
就先只说说E吧。
这个an event that XXXX，是个同位语，同位对应的是an asteroid bigger than Mount Everest slammed into North America，这个现象或者事件。
在这个that引导的同位语里面，我们又看到一个that，它开启了extinctions的定语从句，那么是这些extinctions标志着XXXXX时代的结束。那当然mark要用其复数形式和extinctions一致。
这个句子层次多，比较迷惑我们，所以我们要理清对应关系。
