Is in the twentieth century a prepositional phrase?

No, "in the twentieth century" is not a prepositional phrase. It's a prepositional phrase.

Here's why:

* Prepositional phrases start with a preposition (like "in," "on," "at," "by," "with," etc.) and are followed by a noun or pronoun (the object of the preposition).

* "In" is the preposition.

* "the twentieth century" is the noun phrase acting as the object of the preposition.

Therefore, the entire phrase functions as a prepositional phrase.