GMAT Sentence Correction (SC) | Re: A brand new question from e-GMAT

I was shuttling between A & B and then chose A following the below reasoning :

@e-gmat

Wouldnt the clauses in bold act as independent clauses joined by comma and not by FANBOYS/other conjunction option? Considering this as an error, I opted A.

Discovered in 1930 and once considered the ninth planet in the solar system, Pluto lost its planetary status in 2006, a development that prompted the International Astronomical Union (IAU) to create a new definition for the term ‘planet’
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