Married Versus Unmarried Couples Differ in Terms of Their Well-Being

Married versus unmarried couples differ in terms of their well-being.Although it could be that being married provides benefits that lead to increases in well-being,it is also possible that this is a "selection effect." A selection effect occurs when

A) groups of people differ not because of something special about their group but because they have chosen the group.
B) researchers do not randomly select the groups of people they are comparing,which results in group differences.
C) couples selected mates based on compatible personalities and "love" versus arranged marriages.
D) couples in one group are more satisfied because they are part of a select group of high-functioning couples.