[单选题]当耐张段长度超过800m时，一个耐张段内加强线接头的总数量安全值为( )。
A.0个；
B.2个；
C.4个；
D.8个。

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 difficult to be unified
B. the United States is a big melting pot in which you can be melt into death
C. the United States is a single, unified nation in which it includes the immigrants all over the world
D. in the United States you can find your previous identities