The stars, some of them at tremendous speeds, are in motion just as th

Hello, warrior1991. The use of being in option (A) does not automatically rule out the sentence. I was expecting a gerundive phrase to then serve as the subject of a second independent clause, but that second clause never came. To be clear, being is used as a gerund in (A), but nothing builds off of it. I see that others have drawn attention to the lack of a subject in the latter portion of the sentence as grounds for dismissal, but it is actually the lack of a verb that keeps us from considering (A) (since the gerundive phrase cannot serve as a subject without a verb). The following sentence, which adopts a related construct to the one above, would be valid. I will take the liberty of drawing attention to the gerundive phrase acting as the subject and the verb that helps form the predicate of the second independent clause.

Everyone who eventually reaches adulthood goes through adolescence, yet being a teenager often proves exceedingly difficult in the moment.

If we break down the latter part of (A) in a similar fashion, you can see what I mean:

being so far away from the Earth that their apparent positions in the sky do not change enough for their movement to be observed during a single human lifetime

Yes, that is right: the entire phrase, embedded clause and all, could be thought of as the subject of a potential second independent clause, but no verb appears to tell us what being... achieves. That is, we need to know what being so far away... does to call the whole thing a proper subject. (Change only operates within the embedded clause.)

I hope that helps. If you have further questions, feel free to ask. Thank you for thinking to ask me about this one.

- Andrew