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问题 单项选择题

在USB结构中，负责产生事务处理的是()。

A．USB驱动程序

B．USB设备

C．主控制器驱动程序

D．主控制器

答案

参考答案：C

解析：在USB结构中，主控制器驱动程序负责产生事务处理。

计算题

（18分）如图所示，直线MN与水平线夹角为60°，其右侧有一垂直纸面向外的范围足够大的匀强磁场，磁感应强度为Ｂ；直线ＰＱ垂直ＭＮ，且ＰＱ与ＭＮ包围的空间有一匀强电场，电场方向平行于ＰＱ。有一质量为m 电量为+q的带电粒子在纸面内以v0的水平初速度从A点飞入磁场，粒子进入磁场t0(t0未知)时间后立即撤除磁场，此时粒子未到达MN，之后粒子垂直MQ边界从C点（图中未画出）飞入电场；随后粒子再次通过C点。粒子在以上整个过程中所用总时间恰为此带电粒子在磁场中运动一周所需时间，粒子所受重力不计。试求：

（1）粒子在磁场中运动的时间t0

（2）匀强电场场强E的大小。
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单项选择题

The United States has historically had higher rates of marriage than those of other industrialized countries. The current annual marriage (51) in the United States—about 9 new marriages for every 1,000 people—is (52) higher than it is in other industrialized countries. However, marriage is (53) as widespread as it was several decades ago. (54) of American adults who are married (55) from 72 percent in 1970 to 60 percent in 2002. This does not mean that large numbers of people will remain unmarried (56) their lives. Throughout the 20th century, about 90 percent of Americans married at some (57) in their lives. Experts (58) that about the same proportion of today’s young adults will eventually marry.

The timing of marriage has varied (59) over the past century. In 1995 the average age of women in the United States at the time of their first marriage was 25. The average age of men was about 27. Men and women in the United States marry for the first time at an average of five years later than people did in the 1950s. (60) , young adults of the 1950s married younger than did any previous generation in U. S. history.

52().

A.potentially 

B.intentionally 

C.randomly 

D.substantially
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