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Sentence Correction: Parts of Speech - Articles

The amount of personal vehicles being used not only causes pollution but also affects the economy.

This answer choice is grammatically incorrect. The article the amount of can describe non-count nouns only whereas the noun vehicles is a count noun.

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

much, (a) little, less, the amount of

Although this answer choice corrects the original Parts of Speech error by replacing amount with number to match the plural count noun personal vehicles, it is grammatically incorrect. In the GMAT, not only must be complemented by but also, not just but

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

not only... but also...

Although this answer choice corrects the original Parts of Speech error by replacing amount with number to match the plural count noun vehicles, it is grammatically incorrect. The same part of speech that follows not only should follow but also. In this answer choice a noun (pollution) follows not only, while a verb (affects) follows but also.  

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

not only... but also...

Although this answer choice corrects the original Parts of Speech error by replacing amount with number to match the plural count noun personal vehicles, it is grammatically incorrect. The same part of speech that follows not only should follow but also. In this answer choice a verb (causes) follows not only, while a noun (economic effect) follows but also.  

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

not only... but also...

Great work!

This answer choice corrects the original Parts of Speech error by replacing amount with number to match a plural count noun (personal vehicles). It also maintains Parallelism while using the structure not only X but also Y.

amount of personal vehicles being used not only causes pollution but also affects the economy
number of personal vehicles being used causes not only pollution but also affects the economy
number of personal vehicles being used not only causes pollution but also affects the economy
number of personal vehicles being used not only causes pollution but also an economic effect
number of personal vehicles being used not only causes pollution but affects the economy