[单项选择]Historians may well look back on the 1980s in the United States as a time of rising affluence side by side with rising poverty. The growth in affluence is attributable to an increase in professional and technical jobs, along with more two career couples whose combined incomes provide a "comfortable living". Yet simultaneously, the nation’s poverty rate rose between 1973 and 1983 from 11.1 percent of the population to 15.2, or by well over a third. Although the poverty rate declined somewhat after 1983, it was still held at 13.5 percent in 1987, comprising a population of 32.5 million Americans.
The definition of poverty is a matter of debate. In 1795, a group of English magistrates decided that a minimum income should be "the cost of a gallon loaf of bread, multiplied by three, plus an allowance for each dependent". Today the Census Bureau defines the threshold of poverty in the United States as the minimum amount of money that families need to purchase a nutritionally adequate di
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