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问题 计算题

如图所示，光滑圆弧轨道的半径为R，圆弧底部中点为O，两个相同的小球分别在O正上方h处的A点和离O很近的轨道B点，现同时释放两球，使两球正好在O点相碰。问h应为多高？

答案

题目分析：

点评：解决本题的关键抓住两球运动时间相等，以及注意C球运动的周期性．

单项选择题

与广域网相比，局域网具有( )特征。

A．有效性差，可靠性好

B．有效性好，可靠性差

C．有效性好，可靠性好

D．有效性差，可靠性差
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单项选择题

True, some of the attacks against whites by blacks are for money and valuables. But, others ale revenge assaults by blacks for a real or imagined racial insult. It is equally true that the vast majority of violent crimes against whites are committed by whites, while the vast majority of violent crimes against blacks are committed by other blacks. Yet even after discounting crimes that are hastily and erroneously (错误地，不正确地) tagged as racially motivated, many blacks do attack whites because they all white. A 1998 Justice Department study confirmed that nearly 20% of the more than 4,000 hate crimes well committed against whites, and their attackers were blacks. The Southern Poverty Law Center also notes that there is a huge escalation in black-on- white violence from nearly none at the beginning of the 1990s.
A collection of white supermacists and rightist extremist groups have eagerly made black-on- white violence a wedge issue in their movement to paint blacks as the prime racial hate mongers in America. The New Century Foundation, an ultra-conservative think-tank, has launched a full- blown national campaign to alert whites to the danger of black hate crimes. These groups and individuals magnify hate crimes by blacks in order to back their social agenda. They oppose affirmative action programs and per hate crime laws. They downplay, or justify, the proliferation of white-supremacist-tinged paramilitary groups, police violence, and racial profiling. And they lobby hard for more prisons, police, and tougher laws.
Black-on-white violence also reinforces white fears of blacks as the ultimate menace to society. When blacks say or do nothing about these attacks, it is taken by some as a tacit signal that blacks put less value on white lives than black lives. This is a terrible price for black silence on hate crimes committed by blacks.

The best topic of this passage is ______.

A. Black and White
B. Black-on-White Violence
C. Security Is an Illusion
D. White Supremacists and Rightist Extremist Groups
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