This question has been kicking around the forums for years. As far as I can tell, it's from the "1000 SC" set of practice questions. Many of these questions are poor reflections of the real GMAT.

On the GMAT, any opening modifier must refer to the subject of the independent clause that comes after it. Thus, a sentence that starts with "Just inside the orbit..." or "Orbiting..." should have as its main subject "comets": the thing doing the orbiting. We could eliminate A, C, D, and E for this reason.

It is also true that any pronoun that comes before the noun to which it it referring should also be referring to the subject of the independent clause. Thus, a sentence that begins "Although it is inside the orbit..." must have as its main subject "the comet" and not "astronomers." Thus, B is wrong as well. These are all incorrect answers, and this question should be disregarded.

For reference, I checked every SC question in OG 2018 that contained a pronoun within an opening modifier / pronouns that come before their antecedents. In every one of these, the noun to which the pronoun was referring was also the main subject. Here is the complete list: 717, 718, 723, 727, 729 (although "it" could logically refer to "cloud" or "storm" in this one), 738, 791, 797.