Given the task definition and input, reply with output. Given scene/plot descriptions from movies or TV shows, and given a specific question about the scene, summarize the question into a title

As we all know, the most exciting part of seeing a movie is the trailers of upcoming movies before the actual film (note, this is coming from an American point of view, I am uncertain if this occurs in theatres everywhere).  However, there doesn't seem to be much of a forethought given to what trailers are shown, besides making sure big movie trailers are shown before other big movies. For instance, the idea of seeing a Dark Knight Rises trailer before Avengers (two different production companies, let alone two rival comic book companies) doesn't seem to make much sense to me.  Is there a strict guideline trailers are released controlled by the MPAA or do the theatres have free run of what to show?

What factors go into determining what trailers are played before movies?