Hide the modifier a.k.a fluff and the answer becomes apprent.
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 only correct phrase is Quasars are believed to be the cores of galaxies in an early stage of development.
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 only correct phrase is Quasars are believed to be the cores of galaxies in an early stage of development.
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