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问题 单项选择题

数控机床中的半闭环控制系统与闭环控制系统在结构上主要区别是( )。

A．半闭环控制系统没有位置检测反馈装置，而闭环控制系统具有位置检测反馈装置

B．半闭环控制系统采用直流伺服电动机作执行器，闭环控制系统采用交流伺服电机

C．半闭环控制系统的位置检测器安装在电动机轴端或丝杠轴端，闭环控制系统的位置检测器安装在工作台上

D．半闭环控制系统的速度检测器安装在电动机轴上，闭环控制系统的速度检测器安装在工作台上

答案

参考答案：C

单项选择题

根据所给资料，回答116-120题。

2008年，陕西省全省粮食生产喜获丰收，粮食播种面积4852.1万亩，较上年增长4.4%。总产量稳定增加；果业生产再上新台阶，园林水果总产量突破1000万吨大关。
类别2008年比上年增长(%)
粮食产量1150.90万吨7.8
棉花产量10.07万吨12.2
油料产量49.46万吨26.4
油菜籽33.35万吨23.7
烤烟产量7.14万吨28.5
蔬菜产量1067.12万吨15.0
水果总产量1246.97万吨10.9
园林水果1067.67万吨13.6
苹果745.51万吨6.3
茶叶产量1.60万吨11.3

2007年．茶叶产量与烤烟产量之比为：

A．1:4.7

B．1:4.5

C．1:3.9

D．1:3
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单项选择题

Recently, I flew to Las Vegas to attend a meeting. As we were about to arrive, the pilot announced with apology that there would be a slight (16) before setting down. High desert winds had (17) the airport to close all but one runway. He said that we would be (18) the city for a few minutes waiting to (19) . We were also told to remain in our seats meanwhile with our seat belts fastened (20) there might be a few bumps(颠簸). Well, that few minutes turned into about four-five minutes, including a ride that would make a roller coaster(过山车) (21) by comparison. The movement was so fierce that several passengers felt (22) and had to use airsickness bags. As you might guess, that’ s not a good thing to have happen in a(n) (23) space because it only (24) to increase the discomfort of the situation.

About twenty minutes into the adventure, the entire airplane became very (25) There was now a sense of anxiety and fear that could be (26) noticed. Every passenger simply held on for dear life …except one. A (27) was having a good time！ With each bump of the (28) , he would let out a giggle (咯咯地笑)of delight. As I observed this, I (29) that he didn’t know he was supposed to be afraid and worried about his (30) . He neither thought about the past nor about the future. Those are what we gown--ups have learned from (31) . He was (32) the ride because he had not yet been taught to fear it. Having understood this, I took a deep breath and (33) back into my seat, pretending I was (34) on a roller coaster. I smiled for the rest of the flight. I even (35) to giggle once or twice, much to the chagrin(随恼) of the man sitting next to me holding the airsickness bag.

31（）

A.pilot 

B.baby 

C.guard 

D.man
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