Question about "María y Juan terminan primero"

In this sentence, why is it "primero" and not "primeros"?
Is "primero" an adverb here, considered to be modifying "terminan" instead of an adjective describing María and Juan?
Thank you!

Good question. Grammatical terms aren't my thing, but I'm guessing from the context that there's an implied "primer lugar". That said, "María y Juan terminan primeros" sounds correct to me also. Or maybe "...entre los primeros".  Anyway, hopefully someone who knows what there talking about will chime in...
 

¡Hola VanCC y Steven-W15!

Thanks for asking this question!

You are both on the right track: in this sentence, primero is indeed an adverb implying en primer lugar. This is why it doesn't need to be modified to have an -os ending.

¡Hasta la próxima!

- Liss