Answer:
7. The object of a prepositsion is a noun, noun phrase or pronoun that goes after a preposition to complete its meaning. In sentence C. John found the dog asleep in the barn, the word barn is used as the OBJECT OF THE PREPOSITION IN.
8. Interjections are parts of speech that show spontaneous emotion of the author and could easily stand alone. In sentence D. My, there were a lot of letters on the subject!, the word my is used as an INTERJECTION.