Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They usually indicate location, direction, time, or manner.
* Apart is an adverb that means "separated or distant."
Let's look at the other words:
* Over - indicates location (above) or time (past).
* During - indicates time (within a period).
* Across - indicates direction (from one side to another).