OG11--Q42

[ilovecushi]

OG 11--Q 42
Quasars, at billions of light-years from Earth the most distant observable objects in the universe, believed to be the cores of galaxies in an early stage of development.

the correct answer is (b) are believed to be

i have no problem about the answer and the usage. but i wonder what is the structure of ",at billions of light-years from Earth the most distant observable objects int he universe,"
its not an appositive, right? because an appositive should not start from preposition such as "at". so what's the function of this 插入句?

thanks

[Huang Hsin-Yi]

Quasars, (which is)at billions of light-years from Earth the most distant observable objects in the universe, believed to be the cores of galaxies in an early stage of development

[ilovecushi]

got it! thanks!