Which one of the following is true concerning the definition of social stratification and the stratification system in America?

a) Social stratification refers to the ranking of people into a hierarchy in which the resources considered valuable by society are unequally distributed.
b) Social class ranking is the only form of social stratification in American society.
c) American class system is an open system because people's status is purely based on their achievement only.
d) Social mobility is very high in America as most people achieve higher in social economic status than their parents.