OG11-21

196. It is possible that Native Americans originally have migrated to the Western Hemisphere over a bridge of land that once existed between Siberia and Alaska.

(A)  have migrated to the Western Hemisphere over a bridge of land that once existed

(B)  were migrating to the Western Hemisphere over a bridge of land that existed once

(C)  migrated over a bridge of land to the Western Hemisphere that once existed

(D)
                        migrated to the Western Hemisphere over a
                bridge of land that once existed

(E)  were migrating to the Western Hemisphere over
                a bridge of land existing once

选项E里的exising OG的解释说imprecise,这里为什么用that 从句而不用分词短语，怎么就imprecise了，木看出来？

还有D里的that是修饰 land呢还是bridge 呢？

That 修饰 a bridge of land. 句子要读懂。the Western Hemisphere 不可能在Siberia 和Alaska之间。 我开始做题时也为了赶时间不读其他部分。但是其他部分有很多重要的hint,一定要读懂。

用 ing分词是在形容长久的情况。 once 不是长久，而是曾经一次。

A mountains are in the west, standing between X and Y. 一直就在，而且现在还在。