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问题 选择题

拿两只相同的玻璃杯，一只装水，一只是空的，用一把小勺敲击它们，哪个发出的音调高（　　）

A．空杯

B．盛水杯

C．一样

D．无法确定

答案

答：在敲击时，空玻璃杯振动发声的是玻璃杯，装水的振动发声的是瓶子和水柱，装有水的玻璃杯振动得慢，即振动频率越小，音调低，空杯振动得频率快，音调越高．

故选A

选择题

污水处理池经常有打气装置，使水翻腾．这样做是为了（　　）

A．让污水得到阳光充分照射，杀死其中的细菌

B．做喷泉的效果，让工厂更美观

C．让其中的细菌得到更多空气，更好地分解水中的有机物

D．让其中的臭气充分散发
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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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