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Sentence Correction: Pronoun Questions - Ambiguity

A firefly's bioluminance utilizes more energy to produce visible light than to produce heat unlike an everyday electric light bulb, so it is much more efficient.

This answer choice is stylistically flawed. The pronoun it is ambiguous because it agrees with more than one noun. It is unclear to which noun it refers - to bioluminance or to light bulb (or even, possibly, to energy or heat).

It is true that stylistic mistakes are not always corrected. However, since there is an answer choice that does address the issue of ambiguity, it is preferable to this one.

What helps us identify this question as a Pronoun question as well as identify the ambiguity mistake is the following Stop Sign:

Every pronoun in the underlined part of the sentence or after it

Great!

This answer choice eliminates the ambiguity while avoiding making unnecessary changes in the sentence.

While this answer choice corrects the original ambiguity mistake, it is still stylistically flawed. Replacing the single word so with the longer phrase and the result is that creates redundancy.

While this answer choice corrects the original ambiguity mistake, it is grammatically incorrect. The plural verb are does not agree with the singular subject bioluminance.

This answer choice is grammatically incorrect. The pronoun they can refer to a plural noun, and there's no such noun in the sentence. All the nouns in the sentence are singular.

so it is much more efficient
so bioluminance is much more efficient
and the result is that bioluminance is much more efficient
so bioluminance are much more efficient
so they are much more efficient