The widely accepted big-bang theory holds that the universe began in an explosive instant 10 to 20 billion years ago and has been expanding ever since.
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The sentence talks about what the big-bang theory says. The theory says two things:
- The universe began in an explosion some billion years ago
- The universe has been expanding ever since.
There is no grammatical error in the sentence, and the meaning communicated is also logical. Therefore, the sentence is fine as is.
(B) Incorrect. For the following reasons:
- The past perfect “had begun” is used only when there is an action in the simple past tense before which this action (depicted in past perfect) occurred. Since there is no such sequence in the option, the use of past perfect is incorrect.
- The use of past perfect continuous “had been expanding” is incorrect. We are talking about time till present (“since” stands for time till now). Therefore, we need present perfect tense, not past perfect.
(C) Incorrect. “that has expanded” either modifies “years” or, at a stretch, “instant”. Either of these modifications will yield an illogical meaning.
(D) Incorrect. For the following reasons:
- “the beginning of the universe to have been…” is an indirect way and hence a confusing way to communicate the same idea expressed clearly in option A.
- “that is expanding” doesn’t modify a logical entity.
- The present continuous tense in “that is expanding” is incorrect. We need present perfect because of the presence of “since”.
(E) Incorrect. For the following reasons:
- Error no. 1 of the original sentence
- Lack of Parallelism – “has been expanding” and “to have begun” both attach with “the universe” but are not parallel to each other.