以前的帖子没说清楚啊 og 196

(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

Choice D, the best answer, correctly uses the past-tense verb forms migrated and existed to refer to actions

completed in' the past. Choices A, B, and E present incorrect verb forms for expressing simple past action,

and existing once in E is imprecise. Although choice C manages the correct tense, it misplaces the sentence

elements so as to suggest that the Western Hemisphere once existed between Siberia and Alaska.