DERBY29 wrote: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.
Many thanks.
The word "perfect" is a fancy grammatical term for "past".
has + verb (for a singular subject) and
have + verb (for a plural subject) = present perfect = present past
The present perfect tense is used to describe the present in terms of a past action.
For example, one use of the present perfect tense is to describe an action that started
in the past and might continue
in the present:
Since the beginning of the year, John has memorized 100 GMAT idioms.
The word
since indicates that John started to memorize GMAT idioms sometime after the beginning of the year (
in the past) and that he might still be memorizing GMAT idioms now,
in the present.
In answer choice A, the word
since requires that the verb
has been expanding be in the present perfect tense because the universe started to expand after the explosive instant of the big bang (
in the past), and it might still be expanding now,
in the present.
The verb
began is in the past tense because the universe began
10 to 20 billion years ago,
in the past.
When you're trying to sort out a tense issue, always look for time indicators such as
since and
10 to 20 billions years ago. Such indicators can help you to determine the correct tense.
Hope this helps!
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