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The neutrons residing in the nucleus of an atom have a mass virtually the same as the protons but without an electric charge.
This answer choice is grammatically incorrect and illogical. The comparative construction requires that the things compared be grammatically and logically parallel.
This answer choice is illogical. The comparative construction requires that the things compared be logically comparable. The sentence illogically compares the neutron's mass (have a mass) with the protons (virtually the same as the protons), rather than with the proton's mass.
In addition, the construction A but B (a mass...but without an electric charge) requires that A and B belong to the same part of speech. In this case, A (mass) is a noun, whereas B (without) is a preposition.
This answer choice corrects the original Comparative mistake and logically compares between the neutrons' mass and the protons' mass by changing the same as to the same as that of.
However, the singular possessive pronoun its can refer to any one of the singular nouns in the preceding part of the sentence (mass, atom, or nucleus), thus creating ambiguity and a stylistically flawed sentence. Moreover, the use of the possessive is confusing; The mass should belong to the protons and neutrons but in this answer choice protons illogically seem to belong to the mass.
Similarly, the use of former and the verb do also creates ambiguity. The use of the parallel clauses The mass...is but the former do (A but B) suggests that the former refers to The mass. However, the plural verb do indicates that the former refers to neutrons (the only plural noun in the first part of the sentence). This creates a conflict of meaning due to the flawed style and construction.
While this answer choice corrects the logical mistake in the original question by comparing the neutrons' mass with the protons' mass, it is grammatically incorrect. The construction A but B (Residing...but without [an electric charge] ) requires that A and B belong to the same part of speech. In this case, A (Residing) is a non-conjugated verb, whereas B (without) is a preposition.
This answer choice corrects the logical mistake in the original question by logically comparing the neutrons' mass with the proton's mass.
It also corrects the Parallelism mistake in the original question by replacing the incorrect parallelism of a noun but preposition (mass but...without) to a grammatically correct parallelism of two complete clauses (The neutrons and protons...have but neutrons lack...).
While this answer choice corrects the logical mistake and the Parallelism mistake in the original question, it changes the meaning of the original sentence.
By changing have a mass virtually the same as the protons to Having virtually the same mass as protons do, this answer choice logically compares the neutrons' mass with the protons' mass. By changing have to having and without to lacking, this answer choice creates a correct grammatical parallelism between two non-conjugated verbs.
However, by changing the structure of the sentence, the sentence's main verb also was changed from have to reside. As a result, the meaning and the focus of the sentence change. The original sentence was telling us something about the mass of neutrons as compared to protons. The corrected sentence, however, is telling us something about where neutrons reside.
It's not really a meaning change, but a meaning improvement. Sometimes the correct answer choice must make changes in order to clarify the meaning and intention of the original sentence.
The original sentence is unclear because we don't know anything about the protons being described: the protons is too general to make any sense in the context.
Common sense requires that the things being compared are related somehow. Answer choice D clarifies the meaning by indicating the commonality of the protons and neutrons (their shared location in the nucleus); these two particles are now compared in terms of their mass.
Answer choice E, on the other hand, creates a problematic meaning change that shifts the entire focus of the sentence to the question "where do neutrons reside?" This is not what the sentence is intending to tell us.