Until relatively recently, social psychologists have limited their study of interpersonal attraction to initial encounters or first impressions. why?

The answer is A. Long-term relationship is more difficult to study scientifically.

Now the explanation of a faithful relationship, as stated by Wikipedia, is an "Interpersonal relationship based upon a mutually agreed-upon commitment to one another involving exclusivity, love, trust, honesty, openness, or some other agreed-upon behavior."