GMAT Sentence Correction (SC) | Re: Scientists have recently found evidence

Scientists have recently found evidence that black holes—regions of space in which matter is so concentrated and the pull of gravity so powerful that nothing, not even light, can emerge from them—probably exist at the core of nearly all galaxies and the mass of each black hole is proportional to its host galaxy.

(A) exist at the core of nearly all galaxies and the mass of each black hole is proportional to - "that" needed before "the mass" to maintain the parallel structure
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