Which word is the preposition and which word is the object of the preposition in this sentence?

A great big moth flew through the open window.

Apreposition: great; object of the preposition: big
Bpreposition: through; object of the preposition: window
Cpreposition: a; object of the preposition: moth
Dpreposition: flew; object of the preposition: through

Verified Answer :

Since the only preposition in this sentence is through, the correct answer has to be B) preposition: through; object of the preposition: window.