I have a query in this. How “they” refers to only stars but not planets?
The stars, some of them at tremendous speeds, are in motion just as the planets are, yet 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.
(A) The stars, some of them at tremendous speeds, are in motion just as the planets are, yet being
(B) Like the planets, the stars are in motion, some of them at tremendous speeds, but they are
(C) Although like the planets the stars are in motion, some of them at tremendous speeds, yet
(D) As the planets, the stars are in motion, some of them at tremendous speeds, but they are
(E) The stars are in motion like the planets, some of which at tremendous speeds are in motion but
B is the correct answer.
Often, a pronoun which acts as the subject of a clause refers to the noun which acts as a subject of the previous clause.
A good rule of thumb would be to look at the options. If no options sort out the pronoun issue by replacing it with the relevant noun, just forget about pronoun ambiguity. If there are options that clarify the pronoun issue by replacing it with the relevant noun, consider all other grammatical issues first and then finally zero in on pronoun ambiguity
Here “Like the planets” is a modifying phrase. “The stars are in motion” is an independent clause. The pronoun “they” in the part after comma refers to the subject “stars” of the preceding clause.