Which of the following best explains why people are generally bad at identifying members of different races?
a. Racial bias means that people don't view individuals of different races positively, so they don't invest the effort to distinguish them.
b. Some racial groups vary more in physical appearance than others.
c. Most people don't pay close attention to members of other races.
d. The characteristics we use to distinguish members of our own race are different from those most useful to distinguish members of other races, so we lack practice.