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Sentence Correction: Pronoun Questions - This

Factors such as low salivary buffering capacity, high incidence of bacteria, and an above-average plaque index contribute to an acidic environment around the tooth's enamel that may cause cavities, although this does not occur evenly in all cases.

Incorrect.

Although this answer choice corrects the original ambiguity mistake, it is grammatically incorrect. The plural subject bacteria (the singular form is bacterium) does not agree with the singular verb does.

Incorrect.

Although this answer choice corrects the original ambiguity mistake, it does not fit into the original sentence. The resulting sequence would not does not is ungrammatical.

Incorrect.

This answer choice repeats the original mistake. The pronoun this is ambiguous because it may refer to a singular noun which preceded it - for instance, environment - or to a situation, such as the formation of cavities.

[[snippet]]

In addition, this answer choice is grammatically incorrect. The plural noun Factors does not agree with the singular verb contributes.

What helps us identify this mistake is the following Stop Sign:

A long complex subject / a long distance between the subject and the verb

Did you notice that contribute was changed to contributes?

The lesson to be learned here is that after eliminating answer choices that repeat the original mistake, you should examine the remaining answer choices carefully from beginning to end.

Do not assume that the only thing that changes in the answer choices is the part which was originally flawed. By doing so you risk choosing the wrong answer.

Great!

This answer choice resolves the Pronoun ambiguity in the original question, by specifying that this refers to cavity formation. 

Notice that the restructuring of the sentence does not change its essential meaning. 

Incorrect.

This answer choice is stylistically flawed. The pronoun this is ambiguous because it may refer to a singular noun which preceded it - for instance, environment - or to a situation, such as the formation of cavities.

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.

[[snippet]]

The plural subject of this sentence is Factors - which happens to be the first word in the sentence. What is confusing here is the long distance between the subject and the verb. In this sentence, the words an above average plaque index are not the singular subject of the verb continues, despite their proximity to the verb, but just one item in a list detailing the Factors

If you notice a change, you must also look to see whether this change introduced a new mistake. In this case, it led to a new Subject Verb Agreement mistake.

The plural subject of this sentence is Factors - which happens to be the first word in the sentence. What is confusing here is the long distance between the subject and the verb. In this sentence, the words an above average plaque index are not the singular subject of the verb continues, despite their proximity to the verb, but just one item in a list detailing the Factors

contribute to an acidic environment around the tooth's enamel that may cause cavities, although this
may cause cavities by contributing to an acidic environment around the tooth's enamel, but this cavity formation
contributes to an acidic environment around the tooth's enamel that may cause cavities although this
may cause cavities by contributing to an acidic environment around the tooth's enamel, but this cavity formation would not
contribute to an acidic environment around the tooth's enamel that may cause cavities, although bacteria
Yes, but I thought the change was necessary
No
Yes, but I didn't check to see whether the change led to a new mistake