Time-traveling bees will be mentioned in this post. You've been warned.
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1) First of all, I could not understand the difference between Less than and In less than split in the question. The first one sounded weird, that's why I ended up choosing Answer B.
First, we never want to eliminate an answer choice because it sounds weird! More on that in
this article and
this video.
The distinction here is one of meaning and tense. If I begin a sentence with "In less than 35 years," the expectation is that I'll follow this modifier with an action in the future. For example, "In less than 35 years, Tim hopes to be retired and living on a houseboat off the coast of Spain." Tim's not retired yet, but in the future, presumably, he will be.
In this problem, we're talking about a past event, so "in less than" is inappropriate. However, if we get rid of the "in" we can coherently describe a past action. For example, "Less than 5 years after she learned to walk, Tanya began demanding an iPad with better memory and a phone plan with unlimited data." Here, Tanya's demands are in the past, but they're after another action in the past -- when she learned to walk.
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2) Because of the usage of "had" in the underlined portion of the sentence, it appears that migration happened before the release of African honeybees. The point is, I'm unable to visualize the events on a timeline.
Excellent question! In effect, there are three time markers in this sentence. 1) When the bees were released; 2) 35 years after the bees were released; and 3) when the bees migrated to Southern Texas. We use the past perfect "had" for action #3 because it took place before time #2, which is also in the past. Put another way, imagine that the bees were released in 1900. We know that they
had migrated to Texas before 1935, which is also in the past.
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3) Why exactly is the usage of since in the answer choices incorrect? - since releasing African honeybees - seems to be at a certain point in time [ since a specific event ] .
Take another look at the entire phrase you're referring to:
"In less than 35 years since releasing African honeybees outside Sao Paulo, Brazil, their descendants..."
If you're willing to overlook the problem with the phrase "in less than 35 years," (and you shouldn't be!) there's also a modification error here. It sounds as though "releasing" is modifying "their descendants," meaning that the descendants of the bees somehow released their own ancestors! If someone wants to make a sci-fi movie about time-traveling bees that journey into the past to save their older relatives, I will certainly watch this movie, but on the GMAT, this is considered illogical, so (B) is wrong.
I hope that helps!