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Sentence Correction: Subject Verb Agreement - X of Y Subject

During the process of photosynthesis, some of the light energy absorbed by chlorophylls is stored in the form of adenosine triphosphate (ATP); the rest is used for the removal of the electrons in water, to then be used in the reactions that turn carbon dioxide into organic compounds.

Incorrect.

This answer choice is stylistically flawed. The phrase to then be used is ambiguous and confusing, because it is not clear what it refers to, what will then be used - light energy or electrons?

[[snippet]]

Incorrect.

While this answer choice corrects the original ambiguity by using a relative clause (which is then used) instead of the non-conjugated form to be used, it introduces a change of meaning.

In the original phrase reactions that turn carbon dioxide [...], the relative pronoun that is used to define what kind of reactions these are. By changing that turn to turning and separating it with a comma the phrase tuning carbon dioxide [...] seems to describe the entire situation of using electrons, thereby conveying a different meaning.

[[snippet]]

Incorrect.

This answer choice repeats the original stylistic flaw. The phrase then used is ambiguous and confusing, because it is not clear what it refers to, what is later used - light energy or electrons?

[[snippet]]

Incorrect.

While this answer choice corrects the original ambiguity, it changes the meaning of the original sentence.

The original sentence states that electrons are used in the chemical reactions (i.e. the electrons are a part of the reactions), while this sentence states that the reactions are the ones who use the electrons (i.e. the reactions are those who manipulate the electrons).

[[snippet]]

Excellent work!

This answer choice corrects the ambiguity in the original question by using a relative clause (which are then used) instead of the non-conjugated form to be used, making it clear that electrons are used in the chemical process.  

The corrected sentence also makes the sequence clear: first the electrons in water are removed and then used in the chemical reaction.

Note that in this construction, the complex noun phrase electrons in water is plural (similar to x of y subject), and is correctly modified by the relative clause which are then used, which uses a plural verb (are).

to then be used in the reactions that turn carbon dioxide into organic compounds
which are then used in the reactions that turn carbon dioxide into organic compounds
which are then used in the reactions, turning carbon dioxide into organic compounds
then used in the reactions that turn carbon dioxide into organic compounds
which the reactions that turn carbon dioxide into organic compounds then use