What Is Particularly Unique About the United States Middle Class

What is particularly unique about the United States middle class?

A) It is the smallest class in the United States.
B) It is broken into two subcategories: upper and lower middle class.
C) The people who are middle class often have little to no education.
D) The upper class are as likely to become members of the lower class as members of the lower class are likely to become members of the upper class.