Here's why:
* Prepositional Phrase: It consists of the preposition "in" and the object of the preposition, "conclusion."
* Adverbial Phrase: It modifies the verb of the sentence, indicating the time or manner of the action.
For example:
* "In conclusion, I believe we should invest in renewable energy." (Modifies "believe")
* "The presentation was long, but in conclusion, it was informative." (Modifies "was")
While "in conclusion" might seem like a standalone phrase, it doesn't function as a single part of speech itself. It is a combination of words working together to convey a specific grammatical function.