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Here is a detailed explanation to this question-
thanhmaitran wrote:
Discussion of greenhouse effects have usually had as a focus the possibility of Earth growing warmer and to what extent it might, but climatologists have indicated all along that precipitation, storminess, and temperature extremes are likely to have the greatest impact on people.
(A) Discussion of greenhouse effects have usually had as a focus the possibility of Earth growing warmer and to what extent it might,
(B) Discussion of greenhouse effects has usually had as its focus whether Earth would get warmer and what the extent would be,
(C) Discussion of greenhouse effects has usually focused on whether Earth would grow warmer and to what extent,
(D) The discussion of greenhouse effects have usually focused on the possibility of Earth getting warmer and to what extent it might,
(E) The discussion of greenhouse effects has usually focused on whether Earth would grow warmer and the extent that is,
Concepts tested here: Subject-Verb Agreement + Parallelism + Tenses + Awkwardness/Redundancy• Any elements joined by a conjunction ("and" in this sentence) must be parallel.
• Future actions are conveyed through the simple future tense.
• The simple present tense is used to indicate actions taking place in the current time frame, indicate habitual actions, state universal truths, and convey information that is permanent in nature.
A: This answer choice incorrectly refers to the singular noun "Discussion" with the plural verb "have...had". Further, Option A fails to maintain parallelism between "the possibility of Earth growing warmer" and "to what extent it might"; remember, any elements joined by a conjunction ("and" in this sentence) must be parallel. Additionally, Option A uses the passive voice construction "had as a focus the possibility of Earth growing warmer", leading to awkwardness and redundancy.
B: This answer choice uses the passive voice construction "had as its focus", leading to awkwardness and redundancy.
C: Correct. This answer choice correctly refers to the singular noun "Discussion" with the singular verb "has...focused". Further, Option C correctly maintains parallelism between "whether Earth would grow warmer" and "to what extent". Additionally, Option C avoids the tense error seen in Option E, as it uses the noun phrase "to what extent" rather than a phrase that includes an active verb, such as "is" in Option E. Besides, Option C is free of any awkwardness or redundancy.
D: This answer choice incorrectly refers to the singular noun "Discussion" with the plural verb "have...focused". Further, Option D fails to maintain parallelism between "the possibility of Earth growing warmer" and "to what extent it might"; remember, any elements joined by a conjunction ("and" in this sentence) must be parallel.
E: This answer choice fails to maintain parallelism between "whether Earth would grow warmer" and "the extent that is"; remember, any elements joined by a conjunction ("and" in this sentence) must be parallel. Further, Option E incorrectly uses the simple present tense verb "is" to refer to a hypothetical future event; remember, future actions are conveyed through the simple future tense, and the simple present tense is used to indicate actions taking place in the current time frame, indicate habitual actions, state universal truths, and convey information that is permanent in nature.
Hence, C is the best answer choice.To understand the concept of "Simple Tenses" on GMAT, you may want to watch the following video (~2 minutes):
All the best!
Experts' Global Team
Thank you for your helpful reply.
I believe "whether or not" is idiomatic, but I have seen the use of "whether" alone as in this official example.