Adult contemporary music (AC) is a broad style of popular music that ranges from lush 1960s, vocal-based music to predominantly ballad-heavy music with varying degrees of rock influence. AC radio plays mainstream music excluding hip hop, hard rock, youth orientated RnB, some teen pop music and rhythmic dance tracks (though since the 2000s, some of these have begun to be included), which is intended for a more adult audience. Radio stations playing this format will often target 25-54 year-olds, the group most valued by advertisers. Over the years AC has spawned several sub-genres: "hot AC", "soft AC" (also known as "lite AC"), "urban AC", "rhythmic AC", "smooth AC" (i.e., smooth jazz), and "Christian AC" (i.e., a softer type of Contemporary Christian music). Some radio stations play only hot AC; some play only soft AC; and some play both. Thus it is not usually considered a specific genre of music, since it is merely an assembly of selected tracks of musicians of many different genres.