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Source OG 10:

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

Why is D prefered over A?

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Saurabh Malpani

first, i should point out that this problem is unrelated to the caffeine problem (to which you've given a link). in that problem, both of the competing answer choices are, correctly enough, past-tense constructions (did not seem to bother; did not seem to have...).

here, the situation is not so. choice a uses the present perfect (have migrated), a construction that's inappropriate because the sentence is describing a point event in the past. the present perfect should be used for trends that continue into the present, or to describe the completion of events that still have a direct bearing on some immediate situation being described.

choice d correctly uses the simple past tense: we're talking about a one-off event in the past. as an illustration of this principle, a person's birth is also a one-off event in the past; and one would never say 'my friend joe has been born in 1976.' rather, one would say 'my friend joe was born in 1976.' this is the same deal.

incidentally, you gave this discussion thread a name involving 'past perfect.' be sure you know what the past perfect actually is: in this case, it'd be 'had migrated.' none of the choices in this problem use the past perfect (you are welcome to post another thread if you're unclear about the use of the actual past perfect).

Thanks Ron, I will have more questions on tenses.