There are many girls in my classroom.

Explicação:

REGRA>> Usamos MANY para contaveis e MUCH para incontaveis.

Vamos corrigir as erradas:

I have much books in my room. > I have MANY books in my roomThere are much oranges in the basket. > There are MANY oranges in the basket. I have much dogs > I have MANY dogs

Books, oranges , dogs sao contaveis. > (livros, laranjas, cachorros.) entao correto seria usar MANY

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Bons estudos!