.
Labor force is defined as being the total number of people who are available to work and earn income．This definition includes everyone who is employed or seeking paid employment，so it includes employers and the self-employed．Labor is one of the country's resources which can be combined with other resources to produce the goods and services wanted by the community．
Although the size of the workforce depends a great deal on the size of the total population，there are several other influences which also affect it．The age distribution (分布) of the total population has a very marked effect on the available workforce. If the population has a high proportion of very young people or of those too old to work，then the available workforce would be lower than if there were an balanced spread age distribution．If the population naturally grows rapidly, the number of births greatly exceeds (超出) the number of deaths then as a total population increases proportion of the workforce declines．
Sometimes a population is described as aging which means that the birth rate is either falling or growing very slowly，and as people retire from the workforce there are insufficient numbers of young people entering it to replace those who are leaving it．In that situation the population is top-heavy with older people．So the percentage of the population in the workforce declines when there is either a rapid increase in births or a falling birth rate．
The age distribution of the population has several important effects on the economy．If the population is aging and there is an increase in the number of people retiring without a corresponding increase in the number entering the workforce，this raises the problem of the ability of the economy to provide a reasonable level of social services to the retired group．If the aged are to be cared for in special homes，finance must be available for that purpose．If the size of the workforce is small relative to the total population，then the government tax revenue(税收) is relatively low and either the government has less money available to it or the workforce members have to be taxed more heavily．
72．In the passage “labor force” is defined as ___________.
A．people who are available to work and earn income
B．self-employed people only
C．people who are looking for an employment
D．employers who create job opportunities
73．The conclusion which can be drawn from the second paragraph is that___________.
A．a natural growth of population leads to a balanced workforce
B．a large population does not necessarily mean a higher proportion of workforce
C．the larger the population，the higher the proportion of the workforce
D．the workforce will be lower, if the population has a balanced age distribution，
74．When a population is said to be aging，___________.
A．there are more people retiring than those entering the workforce
B．the birth rate must be growing
C．there should be an oversupply of workforce
D．There are more people entering the workforce than those retiring
75．The population which is top-heavy with older people ___________ of a country.
A．leads to the high rate of unemployment 
B．results in low tax on the workforce members 
C．affects employees’ incomes
D．places pressure on the economy