During the process of photosynthesis, some of the light energy absorbe

Answer B would be correct if water is used in the carbon dioxide process

(A) to then be used in the reactions that turn carbon dioxide into organic compounds
to isn't the right usage it was not intended to do something but rather which was used to do something

(B) which are then used in the reactions that turn carbon dioxide into organic compounds
This is the intended meaning and the clauses are also perfect

(C) which are then used in the reactions, turning carbon dioxide into organic compounds
turninig isn't the right usage it distorts the meaning

(D) then used in the reactions that turn carbon dioxide into organic compounds
which is required after water so that the premise is connected to water

(E) which the reactions that turn carbon dioxide into organic compounds then use
after which what's being is used we cannot figure out

Therefore IMO B

I believe comma which can jump over prepositions like X of Y, which - here which refee X or Y whichever is logical
Education Aisle once shared a pdf on that.
EducationAisle

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How do we know that it is the electrons that are used in the reactions, but not "the rest of the energy" or "water"

alaawaku

Part of that is meaning. Even if we're not scientists, we might notice the logical sequence: First, electrons are removed. Then, those electrons are used for reactions. It's not clear why the electrons need to be removed from water, if the sentence then just says that water is used. We'd need to see something about this altered water in particular. As for "the rest of the energy," it already got used in the first part, so it wouldn't be used again.

There's also the proximity issue. "Which" can definitely modify "water," but it wouldn't go all the way back to modify the subject, "the rest."

However, the most important thing here is that those other interpretations only matter if they lead us to another answer that would then be valid. A and D aren't clear (what is used?), so changing the intended target of the modifier doesn't make any difference. B and C have "are," which only works if we're referring to "electrons." E could in theory be talking about water. However, as with A and D, changing the noun involved doesn't really make E any better. The main issue is the weirdness of saying "which the reactions use." Making the water get used in place of the electrons doesn't really change the overall validity of the answer.