Answer:
The answer is: in my classroom.
Explanation:
A prepositional phrase is the part of the sentence that consists of a preposition and its object. Prepositional phrases can modify the verb or the noun. In the case of the sentence a girl in my classroom threw a paper airplane, the preposition in and the object my classroom, modify the noun, (a girl), therefore it is a prepositional phrase, and because it modifies the noun it is also called adjectival phrase.
