Given the task definition and input, reply with output. Given a sentence and two mentions from the text (arguments), indicate a phrase (a verb or noun phrase) that describes the relationship between the provided arguments.

Sentence: 'Even though Rome is one of Europe 's most populous and busiest urban centers , its spiritual attractions , the churches that tower majestically on nearly every block , easy access to the Vatican , the heavy concentration of monasteries make it a natural destination for Catholics , or anyone else in search of a spiritual retreat .', Argument/Subject 1: 'rome', Argument/Subject 2: 'europe'
be center of