体操运动员在控制体重时应如何进行医务监督？

如图(a)所示，两平行金属板间接有如图(b)所示的随时间t变化的交流电压U，金属板间电场可看做均匀，且两板外无电场，板长L=0.2 m，板间距离d=0.1 m，在金属板右侧有一边界为MN的足够大的匀强磁场 区域，MN与两板中线OO'垂直，磁感应强度B=5×10^-3 T，方向垂直纸面向里。现有带正电的粒子流沿两板中线OO'连续射入电场中，已知每个粒子的速度v₀=10⁵ m/s，比荷10⁸ C/kg，重力忽略不计，在每个粒子通过电场区域的极短时间内，电场可视为恒定不变。求：

（1）带电粒子刚好从极板边缘射出时两金属板间的电压；

（2）带电粒子进入磁场时粒子最大速度的大小；

（3）证明：任意时刻从电场射出的带电粒子，进入磁场时在MN上的入射点和出磁场时在MN上的出射点间的距离为定值，并计算两点间的距离。

Well ____ know him and well ____ know me.

A．I did， he did

B．did I， he did

C．did I， did he

D．I did， did he

(1).p.r.n( )
(2).h.s( )
(3).a.c( )
(4).p.c( )

A.饭前

B.饭后

C.睡觉前

D.必要时

公安工作必须按照( )来实施。

A．党的政策

B．国家法律

C．领导意愿

D．职业道德

Marriage, and its many ups and downs, still exercises a powerful hold over newspapers, magazines and the airwaves. Nearly 23m Americans watched Prince William being joined in holy marriage to Kate Middleton. Millions more have indulged in the break-up of Arnold Schwarzenegger’s marriage after revelations that he fathered a son with a maid.

Less delightful are revelations about the sorry state of marriage across the United States. Data from the Census Bureau show that married couples, for the first time, now make up less than half of all households.

The iconic American family, with mom, dad and kids under one roof, is fading. In every state the numbers of unmarried couples, childless households and single-person households are growing faster than those comprised of married people with children, finds the 2010 census. And the trend has a potent class dimension. Traditional marriage has evolved from a near-universal ritual to a luxury for the educated and affluent.

There barely was a marriage gap in 1960: only four percentage points separated the wedded ways of college and high-school graduates(76％ versus 72％). The gap has since widened to 16 percentage points, according to the Pew Research Centre.

"Marriage has become much more selective, and that’s why the divorce rate has come down," said Bradford Wilcox, director of the National Marriage Project at the University of Virginia in Charlottesville. The project found that divorce rates for couples with college degrees are only a third as high as for those with a high-school degree.

Americans with a high-school degree or less tell researchers they would like to marry, but do not believe they can afford it. Instead, they raise children out of wedlock. Only 6％ of children born to college-educated mothers were born outside marriage, according to the National Marriage Project. That compares with 44％ of babies born to mothers whose education ended with high school.

"Less marriage means less income and more poverty," reckons Isabel Sawhill, a senior fellow at the Brookings Institution. She and other researchers have linked as much as half of the income inequality in America to changes in family composition: single-parent families (mostly those with a high-school degree or less) are getting poorer while married couples (with educations and dual incomes) are increasingly well-off. "This is a striking gap that is not well understood by the public," she says.

Do not expect the Democratic Party, however, to make an issue of the marriage gap in next year’s elections. Unmarried women voted overwhelmingly for Barack Obama. "You don’t want to suggest to someone who isn’t married and has children that they should be married," says Ms Sawhill. "That is a scorn on their lifestyle.

The first two paragraphs suggest that（）

A. the public like to spy on celebrities’ marriage

B. many celebrities’ marriages are going wrong

C. Americans’ marriage is going downhill

D. people feel sorry for Americans’ marriage