简述当前学生失范行为的主要表现。

引起肌肉单收缩的较适宜的电流频率是（）

A.50Hz

B.100Hz

C.500Hz

D.800Hz

E.1～10Hz

如图所示，在做“观察水的沸腾”的实验时：

（1）对比A、B两套器材，你觉得应该选_______套来完成实验，理由是：_________________________。

（2）两组同学绘制和沸腾图像如图C、D所示，对比发现，他们相同点是：沸腾前，对水加热，水的温度_________；沸腾时，水的温度__________（选填“不断上升”、“不断下降”或“保持不变”）；他们的不同点是____________，原因可能是______________。（只要回答出一个理由）

我国为了降低汽车的汽油消耗量，在汽油中加入燃料R，R完全燃烧时发生如下反应：R+3O₂

点燃  .

2CO₂+3H₂O，则R的化学式为（　　）

A．C₃H₆

B．C₂H₆

C．C₂H₆O

D．C₄H₆O₉

完形填空

You have waited 45 minutes for the valuable 10 minutes' break between classes.But when the bell for the next class rings, you can't __1__ how quickly time has passed.      If you are familiar with this __2__, you'll know how time flies when you are having fun and__3__ when you are bored.Now scientists have __4__ a reason why this is the case.      Scans have shown that patterns of activity in the brain __5__ according to how we focus on a task. When we are __6__, we concentrate more on how time is passing.And this makes our brains __7__ the clock is ticking more slowly.      In an experiment __8__ by a French laboratory,12 volunteers watched an image __9__ researchers monitored their brain activity.       The volunteers were told to __10__ concentrate on how long an image appeared for, then __11__ the color of the image, and thirdly, study both duration and color.The results showed that__12__ was more active when the volunteers paid attention to__13__ subjects.       It is thought that if the brain is __14__ focusing on many aspects of a task, it has to__15__its resources, and pays less attention to the clock.__16__, time passes without us really __17__ it, and seems to go quickly.If the brain is not so active, it concentrates its __18__ energies on monitoring the passing of time.__19__, time seems to drag.      Next time you feel bored __20__, perhaps you should pay more attention to what the teacher is saying!

( )1. A. guess     ( )2. A. view       ( )3. A. drags     ( )4. A. thought over     ( )5. A. change    ( )6. A. sleepy       ( )7. A. report     ( )8. A. produced     ( )9. A. so         ( )10. A. partly     ( )11. A. remember     ( )12. A. the researchers ( )13. A. less     ( )14. A. busy     ( )15. A. focus       ( )16. A. However   ( )17. A. recognizing   ( )18. A. enough     ( )19. A. In fact   ( )20. A. in class   B. learn       B. point       B. stops       B. made up           B. develop       B. bored       B. think       B. carried       B. when       B. quickly       B. focus on           B. the experiment     B. no       B. likely     B. gather     B. Furthermore       B. watching           B. full       B. As a result       B. with work         C. believe C. scene   C. backs   C. suggested     C. grow     C. excited   C. decide   C. tried   C. while   C. how    C. forget   C. the clock   C. fewer   C. ready   C. reach   C. Therefore   C. noticing   C. right    C. For example   C. in mind   D. doubt               D. chance             D. gains               D. come up with       D. slow               D. active             D. see                 D. performed           D. but                 D. first               D. tell apart         D. the brain           D. more               D. sure               D. spread             D. Finally             D. counting           D. proper             D. Instead             D. of lessons

根据台地与海岸的关系，碳酸盐岩地台可分为与海岸连接、与海面不连及（）碳酸盐岩台地。