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下图分别是物体运动的速度v、位移s、加速度a和物体受到外力的合力F随时间t的变化

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题型：多选题

题目：

下图分别是物体运动的速度v、位移s、加速度a和物体受到外力的合力F随时间t的变化图象，其中可以表示物体处于平衡状态的是[ ]

A、

B、

C、

D、

答案：

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Passage Two

With unemployment rising and housing costs still high, cities around the country are experiencing a new and sudden wave of homelessness. Shelters are overflowing, and more people this year are sleeping on floors in dingy social service centers, living in cars or spending nights on the streets.
In New York, Boston and other dries, homelessness is at record levels, a consequence of a faltering (摇晃的) economy that has crumbled even further after the Sept. 11 attacks.
A survey by the U. S. Conference of Mayors released last week found that requests for emergency shelter in 27 cities had increased an average of 13 percent over last year. The report said the increases were 26 percent in Trenton; 25 percent in Kansas City, Mo.; 22 percent in Chicago; 20 percent in Denver; and 20 percent in New Orleans.
An unusual confluence of factors seems to be responsible for the surge. Housing prices, which soared in the expansion of the 1990’s, have not gone down, even though the economy has tumbled. A stream of layoffs has newly unemployed people taking low-wage jobs that might have otherwise gone to the poor. Benefits for welfare recipients are expiring under government imposed deadlines. And charitable donations to programs that help the disadvantaged are down considerably, officials around the country said, because of the economy and the outpouring of donations for people affected by Sept. 11.
"This is an unprecedented convergence (集中) of calamities (灾难) ," said Xavier De Souza Briggs, an assistant professor of public policy at the John F. Kennedy School of Government at Harvard. "It’s really a crisis. "
More than half the cities surveyed by the mayors’ group reported that in the last year people had remained homeless longer, an average of six months.
There is no total number for the homeless nationwide. Experts said it was difficult to compare the situation with statistics in previous decades, because counting methods have improved. Yet, several experts said they believed that the increases reported by cities like Boston and Chicago reflected a national trend.
"My impression is there is more homelessness now than there was 20 years ago," Gary Burtless, an economist at the Brookings Institution, said, adding that he believed that economic factors were not the sole explanation.
"I think that there must be a greater segment of our population that has tenuous connections to family and friends, and therefore has fewer resources to fall back on when something very bad happens like when they lose their job," he said.

Which of the following is NOT a reason for the increase of homelessness

A．Unemployment.

B．Housing prices.

C．Sept. 11 attacks.

D．Floods.
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