In the United States the Degree of Individual Income Mobility

In the United States the degree of individual income mobility (that is, the degree to which people move from higher to lower or lower to higher income groupings) is

A) rigid in both directions.
B) flexible in both directions.
C) flexible upward but rigid downward since high income perpetuates itself from generation to generation.
D) flexible downward but rigid upward since most low-income people never rise significantly above the poverty level.