D. cautious about teaching

　　B

　　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 a 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.

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