[单选题]使用验电器前，应先检查验电器的（ ）与被测设备的额定电压是否相符，验电器是否超过有效试验期。
A.工作电压
B.额定电流
C.安全电压
D.额定电压

The economy of a nation requires{{U}} (46) {{/U}}. Total output must grow if the country is to absorb about 1.5 million new workers who enter the labor force each year and more workers who are replaced each year as a form of technological change. If the nation produced the same level of output each year,instead of increasing it,people would have fewer jobs,growing unemployment,and a decline in the per-capita(人均的)income of the nation. To maintain or increase the existing standard of living and to prevent unemployment from rising,{{U}} (47) {{/U}}. Higher rates of employment and substantial per-capita output gains seem to occur when the real economic growth rate is over 3%, as it was in the years 1983 through 1988. Recent experience shows that, with a real growth rate of less than 2.5%,the U.S. economy suffers from{{U}}