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Tesla Slumps Below 50% Share of California's Electric Car Market
(www.insurancejournal.com)
This is a most excellent place for technology news and articles.
Are you asking if Rivian is the only company besides Tesla selling electric cars in the US?
No, nearly all major manufacturers sell EVs. Something like 25 brands.
Edit: although a few of those may still only be selling plug-in hybrids. I know Alfa Romeo is one, for example.