How many people are there in the United States in 1987

A．is defined very concretely in every period
B．is difficult to define in specific terms
C．is defined in material terms
D．is defined according to the standard of each decade

A．the poor people enjoy other benefits
B．society as a whole is not well off
C．the poor people may conceal their real income
D．the poor people need less money to support themselves