题干

如右图所示,一束粒子(不计重力,初速度可忽略)缓慢通过小孔O1进入极板间电压为U的水平加速电场区域I,再通过小孔O2射入相互正交的恒定匀强电场、磁场区域Ⅱ,其中磁场的方向如图所示,收集室的小孔O3与O1、O2在同一条水平线上。则                                                                 (    )

A:该装置可筛选出具有特定质量的粒子

B:该装置可筛选出具有特定电量的粒子

C:该装置可筛选出具有特定速度的粒子

D:该装置可筛选出具有特定动能的粒子

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答案(点此获取答案解析)

C

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    For many in the United States, Arab-Americans are an invisible part of the population. Though Arab-Americans as a community have made great contributions to American society in fields from literature to politics to medicine, many Americans know very little about Americans of Arab backgrounds.
    Arab history in the United States goes back to the late 1800’s when large numbers of Arab immigrants first began making their journey to a land known simply as “Amreeka”.
    Historians generally describe Arab immigration to America in two waves. The first wave took place between 1860 and 1924.The first wave consisted of Lebanese and Syrian, and some Egyptian immigrants. These new immigrants, who were mostly Christian, came to America in search of better opportunities. Even the doomed Titanic, which set sail for America in 1912,had close to a hundred Arab passengers aboard. The majority of Arab-Americans today are descendents of the first wave of immigrants; they are the third or more generation Americans.
The second wave of immigrants followed after World War Ⅱ,caused by political unrest in the Middle East. This second wave of immigrants consists of mainly Arab Muslims(穆斯林)and continues to this day.
    Arab-Americans make up 3 million of the population in the United States, according to demographers. And quite different from popular belief, 64 percent of them are American-born. Eighty-two percent of Arab-Americans are US citizens.
Arab-Americans are beyond the national average in both education and income. Education is important among Arab-Americans; 82 percent have high school diplomas, 36 percent have bachelor's degrees or higher, and 15 percent have graduate degrees. The-median(中位数的)average income among Arab-Americans is $39,580, which is higher than the US average.