I think you understand correctly, but in case you do not, let's look at it a different way.
The argument seems to be saying that because the metaphor appeared in the later book, it must have been inspired by the former book.
What is the argument's reasoning? The argument says that the metaphor is so unusual that it's unlikely that the second author thought it up on his own. Thus, the second author must have been inspired by the first.
What's the assumption? The assumption seems to be that there are only two possible causes for the metaphor appearing in the second book:
1. The author thought it up on his own.
2. The author copied it from the other book.
A good way to weaken this argument is to attack that assumption. What if we showed another possible way that the metaphor might have gotten into the second book?
The OA does so. It says that there was an even earlier book that used the metaphor and that could have been the source for both. By suggesting a new cause, (A) weakens the argument.
(B), however, does not suggest another possible cause. It merely says that the topic of the two books were different. This does not, however, prevent someone from taking a metaphor from an unrelated field and applying it in a new context.
For example, many American metaphors come from sports or from war. If someone said, "Getting this new contract would be a touchdown for our company" he is taking a sports idea and applying it in a new setting.
Accordingly, (B) does not clearly weaken the conclusion.
Elias Latour
Verbal Specialist @ ApexGMAT
blog.apexgmat.com
+1 (646) 736-7622
