[单选题]UPS正常运行时,面板指示灯显示( )。
A.市电指示和旁路指示灯亮
B. 市电指示灯和逆变指示灯亮
C. 旁路指示灯和逆变指示灯亮
D. 市电指示灯和故障指示灯亮

Although the United States has been shaped by continual waves of immigrants, Americans have often viewed immigration as a problem. Established Americans often look down on new immigrants. Despite such tensions, economic needs have always forced Americans to seek immigrants as laborers and settlers, and economic opportunities have appealed to foreigners. The vast majority of immigrants to the United States have come in search of jobs and the chance to create a better life for themselves and their families.
In all of American history, less than 10 percent of immigrants have come for political or religious reasons.
Economic immigrants from Europe, Asia, and Latin America have come to the United States voluntarily. Others, most African Americans, were involuntarily transported to North America to do forced labor or to be sold as slaves. Regardless of the reasons they come to the United States, new immigrants typically work in labor-intensive, low-paying, and d
A. the United States is a nation who hates immigrants
B. the United States is a nation who needs immigrants
C. the United States is a nation of immigrants
D. the United States is a nation who is a melting pot