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Sentence Correction: Relative Clauses - One of the + plural noun + who/which/that

A Giffen good is a product that experiences a rise in demand alongside a rise in the retail price although the term refers to one of the economic theories that do not necessarily occur in the real world.

This answer choice is grammatically incorrect. The relative clause that does not necessarily occur in the real world describes the noun that appears immediately before it, namely, theories. Since theories is plural, the verb that follows it also needs to be plural. However, does is singular.

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

one of the + plural noun + who/which/that

Whenever you see this Stop Sign, make sure that the following verb is plural.

Superb!

This answer choice maintains Parallelism by matching the subject (Giffen good) with a noun (product).

This answer choice is grammatically incorrect due to a Parallelism error. Whenever you come across a sentence containing the structure A is/means B make sure that:

(A) A and B are grammatically parallel (i.e., of the same part of speech).

(B) A and B are logically parallel.

In this answer choice, A is a noun (Giffen good) whereas B is a present participle (ing verb rising
).  

What helps us identify this kind of Parallelism error and correct it is the following Stop Sign:

A is/means B

This answer choice is illogical. The subject of the second part of the sentence is the term. A term cannot be an economic theory. We cannot say that a word or term is something or other, but a word or term can refer to/define/describe something or other.

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

The word/term... (it) is...

Whenever we see this Stop Sign, we know that the topic that the question checks is pronouns, specifically using refers to/defines/describes after the word/term (and not is).

This answer choice is grammatically incorrect due to a Parallelism error. Whenever you come across a sentence containing the structure A is/means B make sure that:

(A) A and B are grammatically parallel (i.e., of the same part of speech).

(B) A and B are logically parallel.

In this answer choice, A is a noun (Giffen good) whereas B is a verbal phrase (
a rising).

What helps us identify this kind of Parallelism error and correct it is the following Stop Sign:

A is/means B

is a product that experiences a rise in demand alongside a rise in the retail price although the term refers to one of the economic theories that do
is a product that experiences a rise in demand alongside a rise in the retail price although the term refers to one of the economic theories that does
means rising demands for a product alongside rising retail prices for that product although the term refers to one of the economic theories that do
is a product that experiences a rise in demand alongside a rise in the retail price although the term is one of the economic theories that do
is a rising in demand of a product alongside a rise in the retail price although the term refers to one of the economic theories that do