Astronomical distances are so vast, compared to those that we are accu

Astronomical distances are so vast, compared to those that we are accustomed to, that by the time an astronomical event has been observed, centuries had already been elapsed since the actual event.

The underlined sections starts with has been, the past perfect tense of be. Upon checking each of the options for splits, we see that verb tense is a 2-2-1 split. The vastness of astronomical distances is something that will always exist. The simple present tense is used in scenarios like this e.g. water is wet, not water has been wet, nor water was wet

(A) has been observed, centuries had already been
(B) has been observed, centuries had already
(E) was observed, centuries had already been
Eliminate for above

(C) is observed, centuries were already
(D) is observed, centuries have already
D sounds right (I don't know how to explain that better)

Imo D.
Correct use presents tense and present perfect tense.
A fact is stated by the present tense. No need to use past perfect tense as the effects are still happening.

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